My wife always criticises me for my budgeting, because I just don't do it :lol: If I want something and I have enough money in my account for it, I will buy it even if it leaves me with little left. Some months I spend nothing, some months I spend a lot. I just adjust my every day spending to last until pay day!
Having a fixed budget is pretty sensible and probably perfectly achievable if your'e collecting relatively low cost stuff like loose figures, but when you're collecting things like MOCs or prototypes that can vary wildly in price, it's very hard to budget.
One important thing to add though - I NEVER get in debt. It's just not worth it, and is a slippery slope. Sure sometimes I'll put something on my credit card if pay day is a few days away, but I always clear it every month. I've seen people get in to serious financial trouble over collecting and in one extreme case even get divorced over it, it's just not worth it.