A few months back Staples was selling 500VA UPSs for about $30 and I
snatched up 5 of them! Now all of my workstations and servers have their
own UPS. It seems like 500VA would go a long, long way for an alarm system.
Even if you can't find some on sale, you ought to be able to find 500VA
models for around $50 and no higher than $60. I've never used anything but
Conext or APC and have had good experiences with both.