I replaced the batteries in a UPS, I also use APC brand.  If you can find a decent price on the batteries, go for it.  By the time I got the batteries, shipping, etc. I probably could have bought the same or better new one from Sam's club, which I did later.  I have several units for different uses, consider them necessary for those electronic devices not only to keep them running for short periods of time, but also to insulate power sources from extreme voltage and amperage changes we seem to have frequently.

Unless the price of the batteries fall, or the price of new units increases, I will probably continue to buy a new unit when the batteries on my present devices fail.  I hate those hoarder shows, I save lots of stuff like old UPS devices, in case the price of batteries get more reasonable.  It could be I just haven't found the correct source of batteries at this time. 
Some say I have cryogenics syndrome, as in keeping old stuff in case of changes that will some day make it usable again.