It would also depend on if they had a crane on site to lift the batteries around. Is a guy in a 3 pc. suit going to do crap like that? No! So there will have to be a tech on hand to switch the batteries, at no less than $30 a pop, plus whatever the charge for the fresh batteries. It ain’t gonna fly!
If the guy could change six batteries per hour, the handling charge could cover it at about $3.00 per change, labor. $2.50 wouldn’t be all that bad either.
I see some sort of a sled a small electric jack of some sort that you take out the old one with, and scoop up and place in the new one in with. You could probably do it in about five minutes if it was set up correctly. if you could do 12 changes per hour, the labor cost could drop significantly per change.
WE need a new battery. I propose these little gadgets ~ http://www.gizmag.com/3d-microbattery/27162/ ~ which, when not in use to power your car and lawnmower can be used as cores in photon torpedoes!