This is a favourite subject of mine.
Many of my bikes have been built up from multiple donor basket cases and have taken hundreds of hours to put together, but because I do the labour myself, they usually have cost me less than $3000 each to finish ready to compete on.
In all cases I would have preferred to buy a complete going bike but before the internet and eBay, it was much harder to find bikes and so back then I tended to buy whatever crap I could find in the hope of getting enough to put together a complete bike.
The other thing is that if you want to buy a particular bike, maybe a rare one, you have to actually tell the owner that you would like to buy it if they ever want to sell. I have bought a couple of bikes like that. In each case it has taken a few years from asking to when they sell, but it does work!!! I have no sympathy for people who stand back, never say anything, then whinge loudly when someone sells to someone else a bike they secretly liked.
Simply a matter of saying "mate if you ever want to sell that, please let me know, here is my phone number"