I've seen Simpsons go from 300-600 normally. It's a great game and replay value as Oryk said is purely dependent on your nostalgia for the game.
For example, Capt. America is practically the same game but it would hold no lasting interest for me. TMNT and Simpsons are what I played as a kid and repetitive or not I like them.
Gauntlet Legends...I've seen these go from maybe $400-600 normally. Those cabs are really cool.
GL? Depending on condition maybe $400, a bit more for one in mint condition. Not sure what it is about those cabs but they hold up very well and many of them are in very good condition.
Some things to look out for with the GL:
1) Make sure that you have the fans in the metal cage working -- if they are missing or disconnected and have been run for any length of time the boards will be in a bad state.
2) Have him turn the GL on and keep it on for a while with you there. You do NOT want the 27" medium res monitor to have any problems because it could be major $$$ to replace/fix. After about 30 min the picture should be as sharp as it was when it was turned on. If the picture looks a bit out of focus you'll need a new flyback and probably cap-kit.
3) While you're burning up the 30 min or so, take the time to go through all of the diagnostics in the game menus.
4) If you do buy the game, spend the extra $40 or so and buy a spare graphics card for the game now while they're available. These are a high failure part and it's cheap insurance. You can buy them here from ToplessZ or from a seller on Ebay. When these are gone when/if your graphics card dies you're SOL.
5) When you buy the game, spend a few minutes and pull out the hard drive, hook it up to your computer and use chdman32 to take a snapshot of the hard drive. The hard drives are another high failure item and leaving a snapshot of the game's hard drive image on your computer for when/if the game's hard drive fails is a great backup plan. It can be a hassle finding the right hard drive image for your game once your hard drive fails so why not spend the 20 min up-front and save yourself a lot of hassle down the road?