Slitherine

One possible model for variable resource pricing

Minimum value of resource say 100
Premium - max premium say 300
Profit - max profit allowed say 200
desired stockpile say 20

Buying ( use actual stockpile not desired)
% of stockpile * premium + minimum cost = per unit cost
1 of 1 = 300 * 1 + 100 = 400
2 of 10 = 300 * .2 + 100 = 160

Selling
(1 - % of desired stockpile) * profit + minimum
2 (0 in stockpile of 20) .90 * 200 + 100 = 280
2 (20 in stockpile of 20) 0 * 200 + 100 = 100
2 (8 in stockpile of 20) .5 * 200 + 100 = 200
if the stockpile % is not limitied otherwise 100 would be minimum 2 (28 in stockpile of 20) -.5 * 200 + 100 = 0