There was a really cool math board game I used to play in a similar class. There were dice with numbers and operations and you had to combine them and meet some rules. I forget the name, but it kept me entertained for hours, although I was maybe 8, so it might be too young.
Honestly, if you are really going to be teaching children in the advanced math or gifted classes, I don't think budgeting is a good topic. The math is too simple to hold their attention for too long. I could be wrong, but at that age, I was dissecting sharks, building civilizations, studying medicine from the middle ages, cracking codes, etc. I would have done the bare minimum on a similar assignment so that I could read a book or do something more interesting.
One really fun thing I can suggest is teaching your students statistics and probability in terms of gambling. It will make them feel less like "smart kids" and more badass.