I have quite a number of the "Rare" games. I have all 7 .hack games plus all of the bonus material for them. I also have games for the PSX like Megaman Legends 2 and Valkyre Profile. Ninja Gaiden Trilogy on SNES and probably a few others.

You can always go to http://videogames.pricecharting.com/ and look at how much games are going for on each system.

While games can be expensive themselves. The old NES stuff gets crazy as some people try to buy the games complete with the box and instruction manuals. For some games people are paying $30 for just the instruction manuals. The world has gone mad I tells you. MAAAD!