You'll note that the gem of healing, necklace of fireballs, and siege stone in
DF 1 cost $100, $120, and $110 per energy point of stuff they can do. That's based on Spell Stone, and includes the enchantment cost
and gem cost. That's the general route I would take:
- Find FP cost to cast for one "charge." (Example: Each 3d Explosive Fireball requires 6 FP.)
- Find enchantment cost for a Spell Stone this big at 20 × FP cost. (Example: A 6 FP spell costs 6 × 20 = 120 energy to enchant as a Spell Stone.)
- Price the enchantment. I'd use $1/energy point here, and cheat and use that up to 10 FP, just because I like the idea of Entombment wands. Maybe DF enchanters have a special Spell Stone variant for wands. (Example: 120 energy means $120.)
- Price the needed gem for Spell Stone at $10 × (FP cost)^2 + $40 × (FP cost), and assume that the magical ingredients needed cost this much. (Example: A 6 FP spell calls for ingredients worth $10 × 6^2 + $40 × 6 = $600.)
- Read the sum of these prices as the "cost per charge" (CPC). (Example: CPC is $120 + $600 = $720.)
- Assume charges can be stacked like Roman candle balls or bullets and simply have the item cost CPC × charges. (Example: A wand capable of 100 fireballs costs $720 × 100 = $72,000.)
I'd make recharge cost equal just the enchantment cost. (
Example: A full recharge for that wand costs $120 × 100 = $12,000.)
You can put all the math in one place like this:
Item Cost = [$10 × (FP cost)^2 + $60 × (FP cost)] × Charges
Recharge Cost = $20 × (FP cost) × Charges
Honestly, given how easy it is to afford Magery 0 or 1, learn the spell, cast it, and take a swig of paut afterward, I don't see this getting out of hand. The quadratic effect in there will make really keen stuff, like my Entombment wand, fairly costly.