I just got an APC SUA-750 for $90 refurbished on Ebay. It'll run my system with dual monitors for roughly 55 minutes ($300 new).
 
Stay away from the Back-Ups like the BX series. They have inflated VA and wattage ratings. Tried a BX1000, and it would only run the system for about 8 minutes. Also weighed about half of what the SUA-750 does. Makes sense since it only has about half the battery capacity. We've also had many of the plastic APC's fail on us here at work.