From a supply standpoint, the number of incomplete bicycles generally exceeds the number of good spare parts available today. The degree to which this is true depends on the part, type of bike, etc. You're correct that some of these parts, like some of the frame tanks, are very expensive because the supply of good, original spares is so small now. Buying something scarce a la carte gets expensive.
I'm not cynical about much because I got past my preconceived notions about bicycle projects. At one time, many years ago, I was dead-set on total originality and period correctness. The more old bikes I worked with, the more I became of the opinion that each bike is just an individual case. Sometimes a wooden tank is totally fine and it works great, and sometimes the bike warrants the big bucks for an original. Sometimes having more gears and aluminum rims is what you want, and sometimes original steel works best for the project. There aren't very many "hard and fast" rules in my book any more. There are a few for something that of extraordinary historical significance, but I don't own very much in that category anyway. Each bike is an individual case, and I don't make any final decisions until I see what I have and get a feel for the project as a whole. My grandfather's heirloom ballooner might get different treatment than a basic, commuter 3-speed might get, for example. So I don't stress too much anymore, I guess. I look at the bike on the stand and start envisioning what I do or do not think works best for each.