Consider APC's 350va unit. You can buy it new for $37. If you need a replacement battery, its $29 plus shipping. In that case, probably buy the new one.
Another common one is APC's BR900va unit. You can buy it new for $85. The APC replacement battery pack is $79. ElcheapoStore.com sells mystery batteries for $55 delivered. Thats not cut and dry, either... I would probably choose the new one as well.
However, there are a few that wont leave you in that Buridian perplexity... the BE750 sells on the street for $82. When it goes bad, the RBC2 pack costs only $29 to replace.
As I told a customer this weekend, replacing the batteries on small APC upses usually isnt worth it, but there are few exceptions.
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And remember, feel free to email me if you are stuck with one of these and staring at those two bushels of hay