I just came out of my Dark Ages so I just blew a couple thousand dollars on current Lego sets, recently retired sets (3x Maersk, 2x Horizon Express), and some classic sets on eBay to catch some "sets that got away" when I was a kid in the 1990s.
In short having an adult income to apply to my childhood hobby has been very rewarding and almost embarrassing! (altho actually, I find it very spiritually calming, and it's surprising how many other AFOLs exist when your Fiancee thinks it'll be funny to call your hobby out to everyone) But, I think this is the initial expensive part and I'll probably slowdown to bricklink supplementing and occasional Lego released. The good thing is I keep my focus on few themes, I'm partial to City, Trains, Modulars, and Creators that mesh with those 3 interests.
For the budget... right now I'm a bachelor with a good job and not much debt, so for the first time in my life, Lego has actually made me have to pay attention to my money, I used to have more money that I ever spent, and I would just toss that towards a Mortgage that is over half paid off in 3 years, or a vacation.
I told my Fiancée that my Lego budget when we are married would be $200 a month, with the ability to roll over if I don't spend it all. She thought that was fair. I think so too, although I might try to get it to $300 :).