My special interests tend to be quite short lived (at high intensity, sometimes they become normal hobby level interests once the obsessive phase has passed) so I very much try and avoid spending too much money on them when I first get into them. I know there's a high chance I will get obsessed for a few weeks and then lose all interest completely. If I absolutely feel the need to buy things, I put work into finding things second hand, getting things on sale etc, finding free things.
Not always successful, I got really into golf in the summer, I tried not to spend any money on it and managed for a while and then thought it was going to last so bought a load of golf stuff (I did try hard not to go all out-I got second hand clubs, and shoes on sale, and picked up a few pieces of clothing at charity shops)...and then my interest kind of waned and I've not used any of it in months, want to find time to go and play again because I do still want to do it, just it's not all encompassing like it was when I was first into it (where I was watching youtube videos and practicing swings every evening, and desperately searching out places I could practice, and telling everyone excitedly about how I was learning to play golf...) so motivating myself to actually plan to go and play is more difficult. I had a thing for really wanting to do metal detecting at one point, but managed not to spend any money on it (I realised I had no way of actually going out and doing it really-I don't drive-which caused me a lot of angst for a few weeks until the urge passed).
I knit, and that tends to be what I spend most of my money on when I'm going through an intense phase. Not too bad though, I get clothes and accessories and gifts to give to other people out of it and usually manage to set price limits on what I spend on yarn (and I don't get into 'trends' so I actually avoid spending money on expensive 'trendy' yarns and patterns-like r/knitting is full of people who like the same yarns and designers and patterns and I'm resistant to all that).