It has already been established that the typical dosage of “Spice” for a dependant user was approximately 2 grams per 70 kilograms of body mass per day. Of course for the drug to cause genetic changes as seen in the Guild Navigators, it would have to be on a much higher scale.
By way of contrast, a typical Heroin (I am basing the comparison on heroin because it is highly addictive and the statistics are probably a little more reliable than those for Cocaine) addict requires 200-300 mg per day, a factor of 10 difference compared with Melange. There are an estimated 11 million addicts worldwide (https://www.whitehouse.gov/ondcp/global-heroin-market), and a worldwide production potential of Opium of 7,400 mt per year, but an total consumption of 3,700 mt per year (UNODC World Drug Report 2010). (Income from this is estimated to be split 3:1 between traffickers and growers/harvestors.)
That’s 3.7 million kilos per year, to supply 15+ million Opium and Opium derivative users who require only a tenth part of a dosage needed by a Melange user.
Let’s consider the economics of Spice production. On the “open market”, the value is 62,000 Solaris per gram.
The famous Base Calculation:
10 billion Solaris per year / 62,000 Solaris per gram = 161.3 Kilograms per year
In the year previous to the Atreides takeover, the Arrakis yearly “legal” profit was 10 billion Solaris per year. After maintenance, salaries, CHOAM cut, and military expenditures the the greedy Harkonnens probably got a 50% profit margin. (the Atreides hoped to achieve 12-15% profit margin) Using the 3:1 standard profit conversion, this results in a production of only 1300 Kg. per year to supply a universe of addicts.
10 billion Solaris / 50% / 15,500 Solaris per gram ≈ 1300 Kg.
Even if the production estimate is doubled to take in the illegal aspects, (smugglers and Fremen), this still results in a production of only 2600 Kg. per year, an amount the DEA would proudly display as results of a single good raid.
An addict such as Piter de Vries consumes 2 grams a day (minimum) times 330 standard days a year is 0.660 Kilos per year. With an entire universe of addicts and casual users, (and remember, the Navigators are probably consuming 100 times this amount) how does 2600 Kg. a year supply the demand?
[EDIT] Putting this into perspective...
Let's say that the wholesale price is a ridiculous 10 bucks a gram (the wholesale price for cocaine where I live);
10 billion bucks / 10 bucks a gram = 1000 mt of powder,
...which is is still less than total opium production to satisfy one sad little planet of addicts.