Prices of resources on the black market vary according to supply and demand.
Not a bad idea, I quite like it.
You can make a handy lump of cash when selling at a high price, and buy resources on the cheap when the market crashes.
But should there be a fixed limit to the maximum price?
At the start of the game, the price you pay for resources has a significant effect, the max price can seem very high.
The variation of prices has the desired effect on how much (and when) you buy & sell.
But mid game the max price becomes less important.
Later on, with a large map, your economy can generate so much wealth that resource prices are not important - you can always afford it.
In fact you can buy virtually as much as you want and the price will not increase beyond the limit.
Perhaps the limit should depend on the strength of your economy or the size of your empire:- I would imagine prices on a black market don't just reflect supply & demand, but also how much the 'seedier elements of galactic society' think you can afford.
Each faction would pay a different price, depending on how rich (and how desperate) they are.
If a faction keeps buying resources at a high price, then their price increases :- the black market senses an opportunity to make a large profit from this guy.
The price for other factions remains high, but doesn't increase so that sales to other empires don't completely dry up.
This might help balance the different race's economies - it could be favourable to the Advent they don't have the credit & resource advantages of mid/late game TEC & Vasari empires.