If you build them to flip them, it's not a money maker. If you build to enjoy, to have, to ride, to savor, then it's well worth the expense.
Consider: a fully done up resto-mod done to a very high standard is $8-$15k depending upon motor work. Then you have essentially a brand new, modern-ish bike. To buy "said" contemporary will cost $7-$18k depending upon your tastes.
In the final analysis, a new off the line bike is the same bike any bloke with a check book can buy. BUT, only a dealer can service due to electronics and the modern technology. And they all look exactly alike. The bikes showcased on this forum are, essentially craft-made, updated machines. Each unique, each worth more than the value of its parts. Rides great, service is easy, parts are cheap. And it's a piece of history that today's modern, disposable attitude would sooner discard as a rust heap, than undertake to resurrect.
I like modern bikes too, don't get me wrong. But for building, working on, sharing time with, I'll take a pre-84 bike from any company all day long.
$ Spent: 550 is >$9k. Had no clue
Budget for 500 is $8.5k got a clue, and probably close
Budget for R90 is $6k I know I can do this one.