A team in Mexico City would work and money is the least of the NFL concerns. Mexico City is in the top 10 of the richest urban agglomeration in the world and there's easily more than 2 million NFL fans.
It doesn't matter if tickets are at $200, $300 dls. or more people will pay it and the stadium will be always full because there's only 8 games per year.
Plus if you add that if Mexico has a team fans from all over the country will be coming in the weekends and will be buying tickets whatever the price. This is a country with almost 30 million NFL fans and by data from NFL Mexico there's 10 million hardcore NFL fans.
One of the biggest issues is that all the stadiums in the city are old and not up to NFL standards. They need to rebuild Azteca stadium or build a new one, it doesn't have to be 100,000; with 80,000 it will be more than enough.
The other issue is why bring a team to Mexico when the fan base is already big, loyal and strong and the market is already here and besides that people with money can always go to Dallas, Phoenix, San Diego etc etc for a game.
A team in Mexico City is more ideal than London even if you take in account economics, but I think that Toronto or San Antonio are even better options.