The Grand Army of the Republic and The Separatists army both had their own advantages (All clones could do independent thinking as opposed to few selected presumably expensive droids, but Droids were easily replaceable which were produced speedily with combat skills). While choosing an army, was cost a deciding factor for a faction? Was one Army more expensive than other?
I would have to lean towards the droid army. The development of all of the systems, vehicles, let alone production, must have cost incredible amounts. Also, the sheer amount of them was large, and they were in service long before the clone wars for the various Unions that had them. The wiki states that a Lucrehulk class battleship can hold over 329,000 B1 Battle Droids. Research on the production of Lucrehulk class battleships shows that at the low end 1,000 were produced and at the high end 24,000 were produced. So, assuming that every Lucrehulk was fully loaded(a fair assumption given the might of the organizations that were supplying droids), and looking at a mid range figure for the production of Lucrehulks (10,000), you would have a B1 strength of 3,296,000,000. You would also have 15,000,000 vulture droid starfighters, 500,000 C-9979 landing crafts, 62,500,000 AATs, 5,500,000 MTTs, along with the other droid types (B2 Super Battle Droids, Droidekas, MagnaGuards, and so on and so forth.) You can see that there was an unimaginable amount of droids that the Separatists had to play with. Take note that this was just with 10,000 Lucrehulk Battleships, and the Separatists have many other sources of droids and cruisers and frigates that all can carry droids and that did carry droids. But, to show you the hopelessness of the Republic, we will use the B1 strength in only the Lucrehulks. The actual figure was probably in the trillions.
The original order for clones was 3,000,000 and another order was made later in the Clone Wars. However, they were not all produced during CW, and Lama Su tells Kenobi that they have created 200,000 units with millions more on the way. So, even if we are generous and include all 3,000,000 clones in the cost, then a clone would be about 110 times more expensive than a droid for the clone army to be more expensive than the droid army.
Sorry, did I say 110? I meant 110,000. Yeah. A droid's blaster alone costs 900 credits (Thanks, Wookiepedia). So, a generous estimate for the cost of a B1 would be 2000 credits. So, for the clones to be more expensive than the droids, each droid would have to cost 219,733,333 credits. If you are not from the Star Wars Universe, that is a lot. Like a lot. Like buy a planet a lot.
Basically, per unit, for the armies to cost the same, a droid would have to cost 219,733,333 credits. Now, REMEMBER, that is just considering the B1 droids that were on 10,000 Lucrehulk battleships.
For the next part of the question, whether cost was a deciding factor, the answer is no. Since no one knew that Palpatine had purchased the clone army (see the bit below), cost was not a deciding factor. And, for the Seps, since the organizations that produced battle droids were part of the Seps, cost was not a factor.
I’m pretty sure that’s it, feel free to suggest any edits. Hope it helped!
Note: In my cost calculations, I did not begin to account for cruisers/starfighters/frigates. The amount of math would be incredible, seeing all the types of spaceships, but it is fair to say that the Seps would still leave the Republic in the dust in quantity.
To the comment above, (I don't have 50 reputation), when Palpatine became Supreme Chancellor, he took funds from the public and sent them to Kamino secretly, so the cloning facilities got paid by The Republic, and since Sifo-Dyas had ordered the clones, it looked good for Kamino.
Clones had blaster rifles worth 1,000 credits, a backup blaster pistol worth 500, armor worth about 4,000, and their units often included troopers with more specialized, read:more expensive, weaponry. All this doesn't begin to account for the cost of the clones themselves. At minimum, a growing clone would have required about ten credits per day of sustenance, for a total of 36,500 credits per clone for a full ten year production run. The clone also requires a roof over their head, invoking the associated heating and electricity costs to provide that living space. They also require training, which Jango Fett was not the only individual to provide. Estimating the cost of said living space for about half the clone's growth (After all, they can't be in cloning vats during training) at about 50 credits per day yields a total cost to house the clones at the facility of approximately 91,250 credits. Then, we have to take into account the premium paid to the bounty hunters who trained the clones. Professionals, who can easily command fees in excess of two thousand credits per week, mount up. Estimating the fees of the two known bounty hunters to replace Jango at exactly 2,000 credits per week, yeilds a total sum of 1,040,000 credits per batch of clones. While this seems exorbitant, for every batch of 200,000 clones, it amounts to just over five credits per clone. When we take into account that it has been stated that an initial payment of one billion credits is required to even begin the production of a batch of clones, and recognize that at the total cost calculated thus far such a payment would cover the production of only 7,500 clones to completion, we begin to see the true cost of the grand army of the republic. If the Kaminoans are to make any money off the arrangement, they'll need to charge enough to cover the 133,255 credits per clone that we've calculated thus far, as well as the cost of staffing and powering the facilities that grow them. Based on the observable characteristics of each cloning disc, we can count 80 clones per disc. This means that each batch of clones requires 2500 such discs. At minimum, this would require 7,500 kaminoan techs to oversee the developing clones, for a period of five years, when the clones are ready to begin training as soldiers. Like the bounty hunters that train these clones, the Kaminoan cloning specialists also command a premium, though perhaps not as great as the bounty hunters. I will estimate the payroll cost associated with each Kaminoan at 1500 credits per week. For three full shifts of oversight, the Kaminoan workforce would need to be paid 2,925,000,000 credits. This works out to 14,625 credits per clone trooper.add this to our precious figure and the total cost per clone comes to 147,880 credits. A single 200,000 clone strong batch costs a grand total of 29,576,000,000 credits. But wait..... this is just the cost of creating the clones! The Kaminoans need to make some kind of money off this transaction! Here is where I'll apply my knowledge of business and recite the little bit of trivia that the amount of money businesses actually make in pure profit is only about 3% of what they take in at the register. If we apply this bit of information to this problem, and increase that number a bit because, given the Kaminoans exclusive knowledge and proficiency at cloning, and the fact that they've quite simply got the market cornered, we'll say the Kaminoans are, rightly, making ten percent profit for their feats of cloning industry. Which means the price per batch of clones rises to 32,533,600,000 credits, with a price per clone of 162,668 credits. This means that the cost of the entire three million clone order rests at 488,400,000,000 credits. By comparison, B-1 battle droids cost 1,800 credits, and B-2 Super Battle Droids cost 3,300. Droidekas, on the other hand, cost 21,000 credits. However, even at that price point, a droideka that kills off a clone has already earned back its cost in attrition rate inflicted on the enemy nearly eight times over. Ladies and Gents. The republic by far fielded the more expensive army. They also destroyed a ridiculous number of droids because their soldiers were of a much higher quality. In the end though, each clone trooper would need to be capable of killing 91 B-1 battle droids, 50 B-2 super battle droids, or 8 Droideka destroyer droids, for them to be able to outmatch the droid armies. No wonder the republic was going bankrupt by the end of the clone wars. They were losing the war of attrition.