It's possible that if they say they have a budget of $500, that they actually have that amount in the bank. It's there a bigger context to your question?
A budget typically refers to 2 things:
1) a spreadsheet with the itemized costs to make a movie = "I have the budget ready, let's go get it funded", in this case the may or not have any money -- but the word budget is used in a different context.
and/or
2) An allocated amount of money to spend on something = "I have a budget of $1000 to rent a camera", means they have $1000 only allocated to spend on a camera rental.