Not exactly sure what flat rate time for gear change is, but I believe it is right at 4 hours (per axle), to which that $250 would be $62.50 an hour.
Dealerships range from $70-$110hr (some even more) for labor rates, most independent shops are about $65-$100hr labor depending on the service and/or brand.
Labor rates were MUCH different 7yrs ago ($45hr), the rates have gone up, even if the work stayed the same.
$250 per axle is reasonable, and I'd rather pay the experianced guy that does a few swaps a month than the dude who does it once a year when the chance arises.
I don't know any of the involved parties in this piss'n match, but experiance tells me looking for the bottom dollar here isn't nessesarly the best way to go.
lets lighten up a bit
Kevin