Pro Rugby in the USA is an interesting one, at the moment they are barely bigger than your local subborban comp. Crowds of 1.5k and TV audiences less than I Love Lucy reruns. BUT if they can gain some traction in the next 10 to 20 years they will become the premier comp in the world. If they can get crowds of 20k and a half decent TV deal it's not unrealistic to think teams could have a salary cap of 30mil, about triple the NRL or any Rugby comp.
Using the NBA as an example. USA Rugby would be the comp every player around the world would want to play in . Every other comp around the world would loose it's stars. The NRL, Top 14, Super Rugby etc would be like the NBL, players who aren't good enough to make it in America