Zahiti - my suggestion to you would be to check completed auction listings for games on ebay, or what people sell certain games for here.
From my experience, knocking a small percentage (10-20%) off non-fluke (once in a while, a single copy of a game will sell unusually high. this is a fluke) successful sales seems fair.
Games that have continual demand are:
Valkyrie Profile (PS1)
Suikoden II (PS1)
Marvel vs. Capcom 2 (Dreamcast, PS2, or Xbox)
Baldur's Gate Dark Alliance 2 (PS2)
Castlevania Symphony of the Night (PS1)
Shin Megami Tensei Nocturne & Digital Devil Saga (PS2)
Breath of Fire I, II, III, IV (first two are on GBA and SNES, third is on PS1 and Euro PSP, fourth is on PS1)
Grandia II (Dreamcast)
The Legend of Zelda Collector's Edition (Gamecube)
Super Mario RPG (SNES)
Chrono Trigger (SNES)
Shining Force II (Genesis)
Phantasy Star IV (Genesis)
Phantasy Star (Sega Master System)
Gunstar Heroes (Genesis)
Metal Gear Solid 3 Subsistence Limited Edition (PS2)
Final Fantasy VII (PS1, because no one wants to pay $60 for this game)
The $15-20 range is good for a lot of these games.
These are not only in demand, but are also good games (a few are a tad overrated, but they're not bad). This list is but a sample. I wish you luck.
-sailorneorune