$10 an amp hour for m18 seems to be a decent rule of thumb although would argue there probably should be some delineation between HO and XC as HO should have a slight premium in my mind.
Not sure if there is an M12 rule of thumb. For some reason those are always more expensive on per amp hour basis despite being lower voltage and no electronics in the batteries. The Milwaukee profit margins must be higher on those although the knockoffs seem to be more equivalent substitutes.
Pulled the trigger on the trim router deal