Usually more than its worth. Don't let that stop you. Its only money. I found this derelict unloved frame in a scrap metal pile. Turned out to be a '73 Raleigh Super Course with the (sorta) rare Capella lugs. Nearly $1000 later it emerged as one of my favorite bikes. It was my first build, with a lot of research lurking here on Bikeforums. I did all the mechanical work myself, stripped the old spray paint, and paid for frame braze repairs, paint and decals, which was by far the biggest expense, over $500. The parts were all used, sourced from Ebay, swap meets and the LBS back room stash. I'd say, go for it. There is probably nobody less adept at mechanical bike repair than myself, but like I said, with help from the forum, lots of trial and error, it all works great, one of my favorite rides. From scrap pile to Eroica, took just over a year, but I'm slow. All it takes is money and patience, mostly patience.

Found this frame, cracked seat stay braze, stuck seat post, wrong fork, ugly paint, but "free ".

Got it together, almost $1K later.

Good times on Eroica, priceless.