Beags - Love me some Airbus as much as the next person, but unless you're prepared to ditch low-altitude ASW completely, the cost is going to be eyewatering. There are surprisingly few part-number-common structure parts between any civvy 737 and the P-8A.
Funny you mention used P-3s - Airbus is very proud of the P-3As it has just modded for Brazil. As A models, they were replaced by Bs and Cs and retired to Davis-Monthan quite early, and cycle-wise they are now spring chickens compared with the USN's operational P-3 fleet.