There is a arena cap at 20,000 seats.
This means that the arena currently acts as a quasi-psuedo salary cap.
If Arena size roughly equals income stream, then you only ever going to be able to have a team that supports 20,000 seats (of course, throw in price differentials in seats, but ignore that for the time being)
So - 20,000 seats = $X income which only leaves $Y to spend on salary.
Sooooooooo. If you REALLY wanted to implement some kind of cap, you could just cap the arena size based on Division.
Div I - 20,000 cap
Div II - 17,000 cap
Div III - 12,000 cap
etc
etc.
Then it would be up to the player to manage his arena prices and profit after wages.
If Div I teams all had an arena of size 20,000, then essentially, they are already operating under this kind of cap-management system.