Usually, when one enters a new system, and starts buying a ton of games you start to get a feel of which games goes for what. Which games on which systems whose price value shocked you? Or, shocked you not that they were pricey, but more expensive than you thought? And if you bought said game(s), how much did you pay?
For me, I have two SNES games that come to mind, and they're both Capcom games.
-Mega Man 7: Seems very hard to find this for 20 or under in good condition, which I thought it would be going into it. Ironically, I recently found a complete copy for 25 and jumped on it.
-Final Fight 3: Wow. This game goes for a ton more than I thought. It seems to rachet over 30 every time.... luckily, I got mine for a measly $15 bucks. I thought this would be a $20 game, tops. Was I wrong.