Dont' know about the cost of each, check the Humminbird web site in the transducer listing.
I would guess it is a straight swap.
Did you notice the "DI" in the transducer model number?
There are two DI trolling motor pucks, one for the small screen units and another for the larger screen units.
Small screen: XTM 9 DI 25 T
Larger screen: XTM 9 WIDE DI 20 T
In no case should you adapt a transom transducer for trolling motor mounting if there is a puck version available. That is the difference between mounting a unit and installing a unit. An installation is better and takes into consideration the unit's use.