Sell Price= Base sell price*(1+.2*ego modifier)
Buy Price=Base buy price*(5/(5+Ego Modifier)
OR
Sell Price/(1+2*ego modifier)=Base sell price
Buy Price/5*(5+ego modifier)=Base buy price
With a little bit of calculation, you can figure out he base sell/buy price no matter what your ego (although you might be off by a cent or two depending on how small the price is).
Base sell price=amount you can sell for at 0 ego modifier
Base buy price=price at 0 ego modifier
Some random facts:
Your ego score has no effect, only your ego modifier matters.
Difference in sell buy ratio at -4 (9 ego, the minimum possible):25 (you sell goods for 1/5 and buy them for 5 times base)
Difference in sell buy ratio at -3 (10/11 ego, what any mutant who puts no points in gets) from 0:6.25
Difference in sell buy ratio at 5 (25/26 ego):1/4 (you sell goods for 2 times and buy for 1/2 times)
The maximum sell price seems to go up to 19/7 base price (it could vary with other items).
The minimum buy price seems to go down to 7/19 base price (it could vary with other items).
Ego bonus caps out at 9 ego modifier (or 34 ego, for which the bonuses are actually capped somewhere around 8.5 ego (in that it doesn't use the formulas)).
Disclaimer: I only tested this out using on item (datadisk), with one merchant (Argyve), but i see no reason it wouldn't apply equally well to all items from all merchants.
If your playing a mutant tinkerer, I've had a VERY good run so far with Wings, Freezing hands, Sleep Gas, Photo Skin, and Analgesia. 24 ego and int, 14 toughness, started as a tinkerer. Give two artifacts to argyve right away to get another level, put the point into freezing hands. Then just wander about locally, using freezing hands on every flying critter you come across, and freezing hands + wings to kill ground bound mobs without getting hit. Split your points on level up between freezing hands and wings, don't go underground for anything, since that negates your flight, and once you hit about level 6-7, you can probably kite Equimax's to death for massive xp. Just be careful, any neutral flying enemy will attack you if you attack an equimax on the same map as it, so it pays to wander around and snipe all the little flyers first.
Seems to me that the 24 ego is kinda pointless. The only reason that i could see starting with such a high ego is for a mental mutant, but if you are just going to sell and buy stuff with it, you should be raking in more then enough money at lower ego (20 should be enough), not to justify getting the bonus.
For mental mutants yeah, the +2 levels for each mental mutation is pretty strong, and definitley justifies it. But you could have used 8 of the attribute points to make your dude much better then trading returns.