If it's in the corner and the size of a business card, I'd say knock $150 off the price. A new backglass typically will cost you $250 or $300 but there are plenty of people that won't mind the crack and would pay near regular price for it.
Ultimately though what it comes down to is, when you're talking about a game that old there's no set price anyways. It could be $200 and I'd say "I can see that" or it could be $2000 and I'd say "I could see that", prices are all over the place for EM's.