@cgr
Yeah, well, about that... You're going to have to wait til chapter 2.
And from what I've read, the cousin isn't supported by the MC. She's actually a tenant so a source of income. And really, I don't know if they're being too frugal. They're getting delivery for dinner. But what you're seeing is the famous Japanese reluctance to dip into the
savings, rather than not spend the money at all. So he's fine with setting aside money to splurge on a BBQ when it comes in, but not so fine about dipping into the saving that's in the bank. It's the difference between slowing down while running, and running backward.
Like I said, he's racing to the target goal by saving when he can. If his serialization ends, his manga income drops to practically zero. He doesn't get royalty from his past work unless his tankoubon gets a reprint. And if his work was popular enough to get reprints, it wouldn't have been cancelled in the first place. He could get a new serialization, but that's not a given. And the apartment has to rent to make money. In Japan, old buildings are worth exactly the amount the land under it is worth, minus the cost of knocking it down. This is especially for any residence and the concept of buying an used home to live in is not very popular. So even if the land itself is pretty valuable, the value of the property (building + land) usually decreases every year (real estate bubble is long, long gone) and unless he plans for it by saving seed money for a new construction, there will be a time in a near future when he may have to sell the property because it won't generate much income for being so old and rundown. Which will generate a nice lump sum, but then what? Interest rate in for simple saving don't even track inflation. Speculate? Kind of risky, and this MC doesn't seem like that kind of a guy.
So what it looks like is the dude got a plan to save as much money as possible while the money is coming in and he's sticking to his plan as closely as he can and not depending on what might be possible in the future.