The whole thing seems like a complete mess. But according to the Guardian they were 3rd in revenue, making more than Liverpool and Chelsea, with Man City ahead mostly because of a better Champions league run, and Man U just being a juggernaut in terms of income, but also having huge debt due to how they were bought. This is for 2015-2016, but I can't imagine it changed that much in a year.
https://www.theguardian.com/football/2017/jun/01/premier-league-finances-club-by-club
Arsenal really shouldn't be economically outclassed by anyone in the league. And investing in them and actually winning big on a regular basis could dramatically increase their sponsorship income and make them one of the top handful of teams on the planet in terms of value. If you look at the Patriots, winning moved them from a mediocre valued team to one of the top most valuable sports franchises on the planet.
I have no clue how this financial fair play system works, and it doesn't seem like anyone else does either. The wage bill can't go up by more than 7 mil a year, but then there are caveats for increasing income. How hard is it to prove one of those happened? How has Man U, City, or Chelsea not run into these limits? Or have they, and I just haven't heard?