Who gave the prison guards the money to pay for Riddick when he is delivered to them?
The prison planet appears to be a self-contained holding facility, more akin to a private prison than a classic government penitentiary. Note that although the bounty offered by the planet Helion Prime (for Riddicks' capture) is $1.5M, the prison's managers only offer Toombs $700K. The clear intention is that they're willing to risk holding onto him in order to collect the full fee. The difference will go towards their operating costs or personal bonuses.
What's not clear is whether this is part of their normal operations (e.g. that their company is acting as a legitimate holding facility for bounty-hunted criminals while they await collection by the relevant authorities) or something the prison managers have decided to do to earn some extra cash on the side.
Either way, $700K is clearly well within the scope of their discretionary funds. Running a prison-planet presumably requires plenty of petty cash for the purchase of food, fuel, etc.