OfficeDepot.com has the Belkin 500VA UPS system for $20 after rebates. One $25 mail in rebate and a $15 off coupon brings the price down to $20. It's really cheap for a 500VA unit, as I paid about $60 for my APC model. It does not say if it is the serial or USB version of the UPS.

http://www.officedepot.com/ddSKU.do?level=SK&id=262669

Rebate: http://rebateimages.teg-online.com/63069.jpg

Use coupon code "35179279".