How to estimate if an old AC is worth replacing?
The EER (energy efficiency ratio) is a lovely mashup of units (BTU/h)/Watts (ie, 12500/1275 in the questioned case) that will tell you (part of) what you need to know. The rest is up to how much you run it.

It uses 1.275 KWh/h (1275 watts) and that costs you 14.07 cents/hr at 11.04 cents/KWh

Compare to whatever you think you might replace it with - you'd want a higher EER than 9.8, or you could just look at 12500 BTU models and compare watts directly. Less watts costs you less per hour to run - if they are all cooling by the same amount (12500 BTU/hr) then the lowest watts is the most efficient and should have the highest EER as well. 

A low-effort search (ie you might find better) shows a current EER of 11.3 on one similar new unit. If exactly equivalent in BTU/h that would be 1106 watts, or 1.106 KWh/h, or 12.21 cents per hour, or a savings of slightly less than 2 cents per operating hour. 

If willing to step away from the window-unit format you may be able to get much better EER in a mini-split type system, but it will likely cost more up front.