Dirt Cheap:

3 Silverstone 16/64-15 tires. Crusty old junk. They've been sitting outside for a long time. Great for rallycrossing rental cars. 0.25", 0.30" and 0.40" tread depth. $5 each. That's right, FIVE.

7 Michelin 14/62-15 tires. Crusty old junk. Most have been sitting outside. The lucky few have been inside for a few years. Tread depths run like this:
.20"
.25"
.25"
.30"
.30"
.30"
.35"

Also $5 each. Do the math, that's 10 crappy tires for $50. Rallycross the shit out of that rental car!

Pretty cheap:

7 Falken RS-01Ds, Medium compound, 205/65-15. Mounted on Nissan Sentra wheels, 4x114.3, all have tubes. Don't laugh. Two on-stange flats in 3+ years, hah!
Tread depths: 6 have 0.35" tread, one has 0.30".
These tires really last, there's at least one rally left in each tire, maybe 2.

$40 each.

3 Falken RS-01Ds, Soft compound. Otherwise, same as above. Nissan wheels, tubes, 0.35", etc.

Also $40 each.

Cheap:

7 BRAND NEW Falken RS-01Ds, 195/65-15, Soft compound, stored indoors. Mounted on Mazda wheels, 5x114.3. Yea, youbetcha, TUBES, baby!
$90 each (that's $75 for the tires, $15 for the tubes. Wheels are free)

Everything is in Long Beach, CA