i suspect getting married had something to do with it. I used to buy tonnes and tonnes of books, collected soundtracks, assembled troops of figurines and model kit mecha units...etc. But somehow I've become like you as well -- I'd much rather spend 20 bucks on dinner and a movie at home cuddled up with the spouse than another CD or DVD. I married young (22) so I should still be in the prime years of blowing away money, but I'm not. The other day a hobby shop was having a 80% off closing sale and I didn't even go.
I went the other direction. My wife is a natural born materialist. She has a tendency to buy things for me that she thinks I will like which turn out to be things that she would like if she had my interests but had her attraction to object ownership (e.g. anime figures, novelty ThinkGeek keyboard).
EDIT: although I must admit that she nails it sometimes - turns out the under-desk cycle and the rotating push-up handles are portable, useful and multi-tasking friendly.
Don't think so, Chocobean. I've lived with guys for years and it didn't change it at all, regardless of their (very varied) spending habits. And when I lived with this one, too.
I'm 100% sure it's a cultural/environmental difference.
Also, I'm not talking about hobby supplies or stuff I might have a real reason to actually want, either. I could drop $200 at Target without even breaking a sweat. Or buying fun things.