I haven't priced a starter directly from cummins in a while but I remember it being something like $400. The guys with the dodge trucks report having some problems with different aftermarket starters but I don't know any more than having heard a few bad experiences. I have had good luck with rebuilt starters but make sure the shop has a good reputation before buying
In my experience, what wears out in the starter/solenoid for these engines is almost always the contacts and plunger. I have never compared the marine
version with the one in the dodge truck but it is probably worth seeing if the internals of the solenoid are the same. FosterTruck sells improved contacts and plunger for the dodge application for under $40 which would be very wise to do if they are interchangeable. When I pulled the contacts on the starter for the 6BT in my truck, they were like razor blades and had very little area left.
I hope that I am not sending you on a wild goose chase looking for improved solenoid parts
but it is worth it if they will fit. I don't have a marine version of the starter here right now so I can't compare.