37signals has published some great tips about Getting the budget out of a client. Basically regardless of how polite you want to be, being blunt is more likely to work eg:
Try this: When they tell you they don�t have a number say, “Oh, ok. So a $100,000 solution would work for you”� They’ll quickly come back “Oh no, probably something more around $30K.” BINGO: That�s the budget.