Consider a used one. Many UPS use a relatively common lead acid battery like ML9-12, which you can buy for around $25. Many UPS, you can easily change those batteries (not all though, you need to research a bit). Check out Craigslist, find the model #, and search for the owners manual online. You should be able to find the battery info, and the ML9-12's are cheap and plentiful!

I got a Cyberpower 550SL on Craigslist for $20, and a battery from Home Depot for $25 (mail order, but free shipping). They'll even take the old battery for recycling. UPS works like new. I have the NAS configured to get safe after 1 minute. I test it every 6 months. This was almost 3 years ago, still working great. When it stops working great, i'll do it again for another $25!