I dunno. Dollar-wise I've got three decent candidates:

1993 Raleigh USA RT600 Purchase price: $15

Pros: Made in USA! Technium! Alan fork! Cons: It just needs cleaning, bar tape and tires and it's basically done. The 105 brifters came back to life with a half can of WD40. This one is a classic example of the problem caused by actual purchase price vs. fair market value. Any one of us here would have likely paid $100 or more for this thing, if they were interested in this sort of bike. (Anyway, definitely not clunkerish enough.)

1988 Trek 400 Purchase price: $25

Pros: Bought it from a fellow C&Ver. Needs repaint. Needs everything. Cons: Nice a enough frame to build up right, rather than cheap.

1986 Cannondale ST500 Purchase price: $50

Pros: Had actual moss growing on it when found, bent rear wheel, poky old Suntour Alpha 3000 shifting gruppo came on it. Cons: too small for me to ride very far.

I've got an embarrassingly large hoard of pretty decent, free, 27" wheels for those second two. A fairly deep parts bin of grungy, midlevel everything to complete. But it kinda seems like cheating to start with something that isn't truly abysmal. Maybe I'll take some stuff out to the co-op and dig through their basket-case frames and see if I can unearth anything really awful.