I don't think this qualifies as "foppish" clothing budget. I don't think I've ever even bought a shirt over $100 - like I said, I'm not fashionista. I'd rather pay to get mid-end clothing right as opposed to go for the luxuriantly expensive stuff.
Also, "well cut" is a very large gradient. Yes, you can buy a vest from a store that fits decently without any alterations whatsoever - but that's also a long way from fitting you like a glove. The point I was communicating is: if you know and have a regular tailor, you can get the absolute perfect fit from factory wares without much extra expense.
The extra bit of fit has a lot of profound effects that are well worth the minimal amount of money you're spending on it. The clothes become yours. They're not a "size 10" or whatever, they're fit for you. All the minor bits that aren't quite right are made right, and even untrained eyes can tell the difference. Given how little it costs I really don't see a reason not to do it.