I'm not sure that makes much sense - pay is dependent more on the role someone is doing, their qualifications and experience and the industry + location they work in.
Prince2 involves taking a short course - you can pass the exams in a week... I think you're barking up the wrong tree if you're hoping it is something that can define you professionally or give your earning potential a significant boost rather than simply being something that is useful for project managers (and indeed others undertaking projects) to know.
As for what project managers can earn in a salaried role, IME, they tended to be on 80k+ at the junior end and that could extend to more like 150k+ for more experienced ones (this was at a vendor selling enterprise software to financial institutions). As contractors the standard contract rates apply at the lower end... 500-600ish (as per devs and BAs)... this can get quite high for experienced ones, especially when they get into "programme management", that's where you can get some experienced people earning £2000 a day etc...