Try buying part-used batteries on the geek surplus market (helps to be
local too). They're rated at a 10 year life but "real" UPSes often swap
them at 5. I've bought massive cells (twice the size of a car battery
cell) at =A31.50 each in the past, although I was buying a ton or two.

And I wouldn't trust APC kit as far as I could throw it. I've never had
a project with APC kit where they _haven't_ failed in a manner that
boiled the battery and dumped acid either onto the client's carpet, or
into a 19" rack cab full of kit.