Quoted from LTG:Just tell them you wouldn't put a price on another persons game. Let them know if they come up with a price to let you know. And walk.
LTG : )™
Yup. If you speak first you're either going to undervalue it (in their view) and piss them off, or you'll inadvertently offer way more than they want and end up paying more. I've had a few people try this with me, and I just tell them that if/when they come up with a price, let me know.