These are STEAM games. The price is what they are now on the store.
I have: Spacechem [$10]
Counter-Strike: Source [$20]
Serious Sam: The First Encounter Classic (2001) [$10]
Serious Sam: The Second Encounter Classic (2002) [$10]
Recettear: An Item Shop's Tale [$20]
Nuclear Dawn (x3 3-DAY PASS) [Free give away to anybody that requests them.]
-75% EVE Online: Crucible [Don't like EVE or MMO's. Free to anybody who requests it.]
ARCANUM: OF STEAMWORKS AND MAGICK OBSCURA
BALDUR'S GATE 2 COMPLETE
PM me. I'll friend you on Steam, gift me the GoG games, and I'll send you the Steam game.
Just an FYI, most traders will value games at their most recent / lowest sale price. You are not very likely to make a trade based on current store prices. It doesn't seem to make much sense, but that is the way they tend to trade.