I've fiddled with enough bikes and parts over the years (and bought and sold lots of them) that I can pretty much build up a frame and fork from what I have on hand these days. The only thing I seem to have to buy to finish a build are gear and brake cables (which I do in bulk), bar tape, sometimes tubes. I suppose those are all sunk costs, and a wise accountant could probably tell me how much I'm actually spending on each build, but I'd likely ignore that.