I get tires fairly cheap, a couple guys at the flea market come in with 30-40 tires occasionally and I can pick up 2 or 3 matched-set Chang Yao or the sort for $10 a pair, as long as I'm buying a few other bits. The last set I got was $15 which included an almost new Mongoose seat and two excellent condition alloy clip pedals with good straps.

I'm usually in it for the $10-15 for tires, $7 for bar tape, $7 for cables, and worst-case $10 for a chain. If I've got to get some hard parts, I find a junker with components for $25-30 and it'll take me through 2 or 3 bikes needing parts, so yeah, about $47 average plus the normal add-in for the lube, nuts, bolts, vinegar, WD-40, etc that's already on-hand. It's weird, but most of the junker bikes I get have good tires and the keepers usually don't. Unless the tires on a bike I get are almost new, I put new ones on them, then hold the good old ones for my riding bikes.