Asking if they have a budget is not unreasonable...if they say "sure...it's in the ball park of $xxxx" then you know they are serious.
If they say "budget?" then perhaps they might be looking for freebies.
Size, or indeed existence of budget needs establishing very early on. You can waste a lot of time figuring out what you charge only to find out they have no money...never intended to have money and were wasting your time.
It's the golden rule of the freelance; find out what the budget is and then plan to take all of it. Additional services, extras, you name it you sell it.