You're in good company
I made a similar $15 mistake this weekend at a garage sale. Spotted a road bike with skinny tires, brazed on DT shifters and a forged crankset and thought "Hey, if nothing else thats a nice $15 crankset" Closer inspection revealed a Shimano 105 freehub laced to a Matrix rim, and a nice Mavic rim with eyelets up front...Drivetrain was all matching too: Suntour Alpha 3000.
So I ask about the bike and the lady says to me "Oh that thing is pretty messed up, I don't know if you'd want it". I said how about 15 bux and she was all about it. Rode home, got my car and drove down to pick it up and as I'm wheeling it to the car I hear *tink tink tink* from the wheels. Look and its a busted spoke. No big, I can work with that....ah crap its 2 ...no wait its 3 and OH F**** the Hub Flange is cracked
So i've got a set of Suntour Alpha 3000 DR's and shifters, a nice Matrix and Mavic rim, and a 105 rear hub.....that came with an aluminum frame.
not a total loss, but it was a pretty big come down from what looked like a $15 bike that needed some new cables to a $15 parts purchase.