Wow…that garage is stiffing you !!! Or maybe they think that your truck is a Ford and are charing you book rates for the Ford, which is a MUCH more timely and complicated procedure then changing the water pump on your Cummins ISB. Either way, they are NOT to be trusted.
While the oem Cummins water pump, made by Concentric, IS expensive at about $165 through Cummins and possibly twice that from a Mopar dealership, they last MUCH longer then 2 years. But most all other aftermareket water pumps DO tend to fail after a couple years, don;t really know why, I just have to guess that they use cheaper bearings and/or seals. So all that being said, it seems VERY likely thatyour mechanic/garage is charging you dealership prices for the oem pump and then simply installing cheap aftremarket ones which are quickly failing on you…really the is NO other explanation to the failures, as the oem Cummins do NOT fail at that rate.
Like others have said, runaway from them, though perhaps give them a pice of your mind first.
Just my .02