NFL franchises are designed to be profitable. The salary cap’s set at a % of revenue, I’m pretty sure less than 50% of revenue goes to player costs, and pretty much benefits from a system that essentially develops players free of charge as players represent high schools then college teams, arriving in the league pretty much ready to play with a really short shelf life via the draft. Rookie contracts protect teams and the entire system is designed for teams to not be bad for very long. NFL teams are pretty much at their earnings potential, though, whereas football is still considered to have untapped potential yet to be exploited.
NFL teams are rightly valuable, there’s only 32 of them with little chance of expansion, teams get stadia funded by taxpayers and a lot of the teams pretty much represent a state rather than a town/city, meaning their ‘catchment area’ is more akin to international football teams than clubs. All of that stuff will be factored into those valuations but I wouldn’t be shocked if in 10-15 years time the majority of the most valuable sports teams in the world are the big European football clubs, especially if UEFA and the premier league properly enforce rules around proper profitability, which a lot of these American investors will push for.