A rough sketch of what I'd try, in an exhaustive manner:

• Pick a quirk.
• Subtract the cost of materials required to expend and recharge the powerstone ten times.
• Estimate roughly how often your players will be able to use it. Multiply the cost by that fraction.
• Subtract 10% for a minor nuisance effect / non-costly usage requirement, 20% for a major nuisance effect, 50% for something illegal.

Powerstone A is 10-point a powerstone ($35 in your world) that smells of fish and can only be used on Wednesdays. It can be used 1/7th of the time, so multiply $35 by 1/7 and the remaining cost is $5. The nuisance effect is minor; subtract 10% and we get $4.5 (rounded up to $5).

Powerstone B is a 10-point powerstone ($35) that requires burning incense to recharge ($20 for 10 recharges, leaving $15) and can only be used while humming God Save the Queen (minor non-costly usage requirement, -10%, leaving $13.5, rounding to $14).

For a random grab-bag, I'd figure out average costs and divide by two to account for bad luck and having to figure out the quirks yourself.