Here is a good read for you to understand the NFL minimum -
http://www.spotrac.com/blog/nfl-minimum-salaries-veteran-discounts/
And again, it is off topic, but the fact is that the NFL is so powerful that agents could call the top 100 players in the NRL and get their attention, where as NRL agents would struggle to get anywhere with NFL players. I am not taking shots, it is just the way it is based on population, sporting dollars, etc. The MINIMUM salary to play in the NFL is more than twice the NRL Average, so it makes sense it works that way. This only go to show that the domestic fans of RL in the USA not only need to find a reason to support their national team, but they MUST be able to relate to it through domestic players. They need to be able to know some of the faces on the team, or they just find it irrelevant. That is just the way it is.
And as for the 3 other nations you mentioned, what are the other sports they compete with domestically? Very little when it comes to sporting dollars. Again, the USA is full of BILLION DOLLAR sports, so if the fans don't find a way to relate to the players in the teams, it will never be more that it is today. Their MUST be a balance of Domestic players in any national team that represents the USA.
As for Jessie Williams, he was an OK college player who played a tough position for a Rookie, in one of the best teams in the NFL. And I am not convinced he has what it takes to make it based on the players he is up against in Seattle, but I am sure he is making pretty good money compared to a first year NRL player.