Sorry... All I have is 400s...
Note that for $100 you can get an Athlon CPU and motherboard from newegg.com...
TigerDirect.com has the K6-2 500MHz for $ 39.99... for comparison, Newegg has the AMD Athlon Thunderbird 1.1GHz for $33.00 and the AMD Athlon XP 1700+ Thoroughbred for $50.00.
On eBay, there's a K6-2 500 with an $18.00 Buy It Now!that ends in a few days... I don't think you're gonna do much better than that...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2723088796&category=3739(In fact, there are lots of K6-2 500's and 533's on eBay right now... might be your best bet!)
--Chris