This is changing. In states where NPs practice independently, you have to be certified in your area of expertise or this will be required soon. BONs are becoming more restrictive and FNPs functioning in a psych role are becoming less common. In my PMHNP class, there were 3 FNPs doing a psych post master's re-certification because they were no longer being able to function in a psych role and be eligible for reimbursement. Again, this is generally in states where NPs are independent. Texas, for example, is different because NPs have to be supervised, but when you are in an autonomous role, it is not so easy to float between specialties.
As far as salaries, psych NPs tend to make quite a bit comparatively. Here in Arizona, NPs are fully autonomous and practice independently of any supervision or required collaboration. A PMHNP can essentially do everything a psychiatrist can do here and salaries in the $100,000-120,000 range in CMHCs are the norm. The local prisons pay even more. Private practice depends on how hard you want to work. But, again, you have to be a certified PMHNP to get the job - they will not consider a FNP or other speciality. But, make sure that psych is what you want to do because you will have to practice in a psych setting when you're done if you go the PMHNP route.