In a modern setup, the most expensive part of a UPS is the battery. If you like DIY I'd say look at some of Cutting Edge Power's concepts, a sinewave (modified or pure) inverter/charger and a bunch of batteries is pretty much all you need.
There are also a bunch of sealed box solutions (Blue Yetti, that lion mascot one, etc) who I can't personally vouch for but other people seem to be reasonably happy with them.
There are also a bunch of sealed box solutions (Blue Yetti, that lion mascot one, etc) who I can't personally vouch for but other people seem to be reasonably happy with them.