The NFL, like the NHL, has a hard salary cap, so there's a limit that every team is allowed to spend on players, with no exceptions.
hmm, I think it's much more to do with sponsorship deals than the salary cap.
Based on the reported salaries here http://www.dailymail.co.uk/sport/football/article-3913302/Cristiano-Ronaldo-declares-got-10-years-left-Real-Madrid-star-pens-new-five-year-365-000-week-deal.html http://www.azcentral.com/story/sports/heat-index/2016/06/10/nfls-highest-paid-quarterbacks-2016-rankings/85668926/
Ronaldo and Andrew Luck would both be on around £18.98m with current exchange rates. So that's just a huge difference in endorsement money rather than just the salaries that make it up.