I don’t have any fic but I have headcanons because i think about this a lot actually:
Simon absolutely argues every time Baz offers to spend money on him, and Baz never understand it no matter how hard he tries. It’s a point of contention when it comes to take away & groceries especially, Baz wants to spring for the fresher foods, the higher prices, the best he can find, but Simon’s always going to default to what’s cheap and quick and easy.
But also, he has some irresponsible spending habits of his own. He’s uncomfortable having money. If he has it, he feels like he has to get rid of it. He’s not used to it, he doesn’t know what to do with it, he just feels like he has to spend it. Saving doesn’t feel like a thing.
Simon stresses out around holidays and Baz’s birthday as well, because what do you get someone that can have anything they want? When you can’t afford shit? He settles for homemade things but definitely feels like it’s not good enough sometimes. And he gets squirmy and uncomfortable when he finds out Baz has spent too much on his gift.
His clothes? He won’t throw anything away. He doesn’t really know how to mend them, can maybe do a basic stitch, but no matter what he just. Won’t throw out clothes. He definitely wears them down to the threads, maybe gets a new pair of jeans once a year? New trackies even less frequently? He’s definitely still got a couple pairs from when he was a kid. They don’t fit that well anymore, he’s taller and rounder, but he keeps them just in case. (Baz buys him new clothes more frequently, just picks him up a pair of trousers or a shirt he thinks he’d like when he’s out. See again: This definitely makes Simon uncomfortable to a degree, he’s thankful but why would you do this i have clothes?)
Disordered eating? I’d bet money. He either forgets meals constantly because they’re not scheduled and prepared, or he eats the bare minimum because he’s afraid to use up the food he has. Yeah, logically he just goes out and gets more. Logically, he and Baz go to the store at the end of every week. But he’s afraid of the cabinets being bare. He’s afraid of not having it anymore, so he’s afraid of using it.
There are hurdles and things they have to get used to. Baz spends time convincing Simon that he’s worth the few quid that shirt cost, but he also works to be a little more frugal to quell Simon’s anxieties (even though they both know it’s not a problem), he works on other love languages.
They’re in no way deal-breaking issues, but there’s work to be done.