Inexpensive is a relative term. None of them are cheap. You need to say what you need it for and what load is needed, examples:
You may only want time to do an orderly shutdown
You may want say 10 minutes to inform clients you are down
You may want to blast some alien scum into the nearest black hole and check top scores
You may only need your PC to keep running
You may want your printer available for so many minutes
Here for example is an extract for one UPS, battery run time:
Entry-level PC with 19" LCD (150W) - 50 min.
Midrange PC with 19" LCD (300W) - 16 min.
Performance PC with 19" LCD (500W) - 9 min.
I don't use a UPS so I have no specific recommendations, others may.