Yes there is a way to do that. I remember seeing the base values somewhere but I don't have the game installed anymore so I can't look for it. It would be in one of either DefaultGameData, DefaultGameBoard or something like that. You may be able to search the files for "Black Market" or "Intel" and see if it pops up.

This mod should not cause any performance problems.

This only affects buy prices (like, when *you* go to buy things from the Black Market). It does not affect sell prices (when you sell items to the Black Market). I will add this into the description for clarification.

THere's a signle preset value for the average price and the game applies the same supply/intel conversion ratio for selling/buying?