I have bought and flipped two bikes. A third has ended up as a rider (oops).All were bought about five years ago, one was sold quickly, the other two... were covid projects if that gives you an idea of my timelines for getting them done.
1974 Schwinn Le Tour: Bought for $75, probably put about $25-$40 in cables, bearings, other parts (maybe tires? I dont remember, so maybe more than $40) and sold for $150. Made about $25-$50 for a weekend of fund fixing a bike.
1982 Peugeot P8: Bought for $80 (Waaaaayyyyyy overpaid at the time. This was a mess, "indexed" headset, bent derailleur "claw" bottom end to begin with, etc.). Was in better shape than I thought. Just needed bearings and grease in the headset, tires were serviceable, wheels were true enough (next bike mechanic skill is wheel work) etc. Sold for $150 after about 2 months of advertising on Craigslist. Made about $50.
1973 Raleigh Competition: Bought for $50. had no idea what it was at the time or what I was getting myself into. The original TA cranks gave me fits (didn't know they were 23mm) the headset gave me fits (the crown race seat is 27mm, not 26.4mm) the seat post was fused into the frame, no saddle, crappy replacement derailleurs, I decided to repaint...etc. I think I added it up and I am all in at about $350, maybe $400. Pretty sure I would struggle to get that out of the bike. I love the way it rides and I have 700x32 tires on it, so it is also kind of my winter/gravel/go everywhere bike at the moment. Great $400 spent.
So I think the story is: you basically have to find complete bikes that need minimal maintenance (cables, housing, maybe a chain, bearings. Tires eat up budgets FAST) for low prices (I think someone else mentioned $50, and I agree). It helps I am in the Balitmore/DC area. The "$100 bike" in Michigan (I grew up there ) is a "$150 bike" here. That $50 makes all the difference with my flips (so far) in losing money or making money.
You HAVE to jump at the $50 bikes if they look good. And pass on the junk.
If you can get a small stash of 27" tires for cheap ( I think schwalbes are $20 right now from a few vendors) it might be worth it if you're going to do this.
Or, you look for the special bikes and buy them when no one is looking, but that's a LOT of work!
Smiles and Happiness
Splendidutional