The batteries should only be replaced with the unit unplugged from ac power,
the power switch in the off position, and all devices unplugged from the
outlets on the unit as an added precaution (not to mention the extra ease in
turning it upside down without the extra cables hanging off it).

I could see this being true, except there was no load, and the ups is
hot swappable. (Also, on SmartUPS, the batteries slide out the front
of the unit.) There was no load at the time, because I was having
problems with it turning off at random times because it would go on a
self-test and die because of the dead batteries. All that was plugged
in was a monitor, and I unplugged that too because I couldnt see the
screen!

So back to my question. Should I pay $80 for APC to send me new
batteries and a warranty or just go out and buy a all new unit for
double that cost? Perhaps this is fixable, as I have knowledge in SMPS
repair and design, however I have heard that APC refuses to provide
prints for their units.