We planners withhold our budget for obvious reasons (to us, anyway). We think that sharing our budget will lead to abuse of it. For example, if I disclose to a hotel that my budget for room rate is $199/night, I fully expect the hotel to come back with a room rate of $199, whereas if I hadn't disclosed my budget, they might have come back with a rate of $179. Suppliers, I'm sorry, but that's how we think, primarily if we don't have an established relationship with you. If we trust you, it's a different story, and we're partners.
Imagine going to buy a new car, and the dealer asks you your budget. "Well, I'm really not sure I want to share that information with you..." You will be test driving Maseratis and Mazdas, wasting your time and hers. At least giving a range or a ballpark figure would help both of you to be on the same page.
My advice to planners: Give your suppliers a chance to earn your trust. Describing past events can give them a good starting point, and sharing your group's demographics, goals and objectives can help your supplier partners craft a more meaningful proposal. Otherwise, they're throwing darts with a blindfold on.
My advice to suppliers: The first time we work with you, we likely will approach you from a point of distrust. Be patient, explain why it's helpful to have key bits of information, and ask us itf we'll at least share whether we have a Maserati budget, or a Mazda budge.