What's the real "value" price of TF2 keys?

depends if you're selling or buying, the truth lies in the middle :P

on a more serious note: the money the key-seller gets from the market is more important.

Imagine it this way: you have tf2 keys and want to trade with s.o. else for games from the store - why would the other person accept your key as 1.75 when he gets at most 1.53 from selling the key on the market?

for real life cash trades 1.8-1.9$/key on comm market its 2.3$-2.45$ also a seller selling a key on comm market for say 2.3$ will get 2$ after the 15% fee deduction.