Hows a Honda Spree grab ya ?
1986 Spree - $500/OBO - is Listed fer St Louis, MO
You can also keep an eye on http://www.cycletrader.com as well as E-bay.
Check Alla the online classifieds too, ask around - it's possible to find one quickly if yer devoted to doing so, and you can get a pretty good used 'ped for $500-600, long as you don't mind doing a little tune-up work.
(Ok, the Spree ain't exactly a moped, but he said desperate, right ?)