3 P-8 Poseidon base aircraft without weapons doesn't cost much.
Portugal has been operating American submarine warfare aircraft for decades god sake.
God sake what?
P-3Cup+ is good to go for several years...P-8 is not a priority for the moment.
Currently P-3CUP+ is a great weapons system for PoAF, there are other fleets that need more attention than that one.
And in the current mission of P-3CUP+, anti submarine warfare (ASW) is a low percentage of their use. A great use is for SAR missions way beyond other PoAF fleets reach, and for course Surveillance and Reconnaissance...
So P-3CUP+ is well prepared for the current and future operations.
The USNAVY has P-8 because they can...spend millions in upgrading their fleets...