The current tag wiki for the budget tag is:
Use this tag when a major factor in your question involves any of these terms: budget, low-cost, deals, shoestring, frugal, cheap, or sales.
But isn't it just as likely that people would use it to tag questions of how to work out a budget for a trip or aspect of a trip?
I think "low-budget" would be a better tag to fit the description currently in the tag wiki. I have tagged one of my questions with shoestring to indicate extreme low budget (this covers stuff such as hitchhiking and sleeping rough)