On the other hand, surplus rack mount servers are very cheap on eBay. You can certainly stick non-rack UPSes into a rack. I did this for about a year (two APCs sitting at the bottom of the rack) before I got a great deal on some surplus rackmount UPSes.
Yeah; buying new is extremely expensive. Many cities have an e-waste recycler from whom you can get great deals (otherwise browse Craigslist and Marketplace, there are good deals (sometimes free) on older equipment.
For UPSes, a reputable brand (like APC) will still function fine but might need new batteries, which are a lot cheaper than a whole UPS.
That's a great point about a second-hand UPS. My mind has been tainted by all the non-removable batteries inside consumer products so I completely forgot that a worn-out battery on an old UPS is easy to replace.
I haven't shopped rackmount UPS before but for lower end consumer APC UPS the official batteries from Amazon or the APC website are often the same price or more expensive than a whole new APC UPS from a slickdeals-spotted sale.
