Just my 2c, I always wait for the question "What is your budget" and my answer is the same "none... I am open to get this piece done" which is 98% of the time followed by the artist hourly rate.. I want the best art been performed with no restriction and definitely I don't want to be the cause of what I want to be restricted by myself...
After that I than proceed to budget the hours I can afford at the time.
BUT
When in doubt, I ask :)