MLS is experiencing their most financial success to date though, with many teams investing in new stadiums and reaping the benefits of that. Ticket sales have seen significant growth recently, and that was before Beckham signed. There's money in the sport in the US, and it's a financially viable league, unlike NASL. Yeah, it's a lot of money to pay one player, but believe me, these people have it, or they can get that kind of money.
Here in New England, the owners of the New England Patriots, the most successful American Football team over the last 6 years, also own the local MLS team, which has also enjoyed some success. It's not their style, (they're known for drawing a hard line on player salaries) but I'm sure they could invest a hundred million or more into a soccer player if they wanted to.