steam trading cards exchange for games

I think you should go for the price after the tax, since the guy isn't receiving the selling price. I'd say it's pretty fair, but it depends on which game you're trading for as well (sometimes you might be able to get the game for a cheaper offer).

it depends on what game are you trading for and how much you want to have it..

Depends on sellers, honestly. I usually go for the average between the listed market price and the price after fees. (Or, in other words, I let the other person keep half of the theoretical fees.)
Some calculate strictly by market prices, some strictly with prices after fees. It all depends on what you previously agree upon, although the default in my experience is full market price (with fees).
Just always make sure to use Steam Inventory Expert to check the traded amounts.