Jason at 37 Signals shares this tip he received from his friend Carlos:

First you should just ask them for it. Be blunt: “What’s your budget or budget range for this project?” If you have a good client they’ll tell you (and trust me, you want good clients — finding the right clients is 90% of this business). If you have a reticent client they may say “we don’t have one yet” or “we’re just looking right now” or “we want you to tell us how much it will cost.” Truth is, everyone has a number in their head. They have a good idea of what they can spend or they wouldn’t be shopping in public. If they don’t then they shouldn’t be asking you to invest your time in writing a proposal — and you most certainly shouldn’t provide them with one.

So, how do you get the number when they won’t tell you? 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.