Couple of things..
The $250 Million is a combination of salary, promotions, endorsements advertisement, etc... That is not all going to be paid by the L.A. Galaxy.. Still, that is a crap load of money...
Second, as probably mentioned, while Beckham is not the best player in the world, or for that matter, even close to being the best player in the world, I believe he will now become the best in the MLS.. He himself cannot make soccer in the US popular but the hope is that other stars from Europe will follow and through time the league will grow in skill.. His arrival will have a very short term effect on interest but in order for interest to maintain and grow, more players of high quality will have to follow....
And I know there are a lot of them, but I don't believe "This is America, we will never like Soccer" because I'm American, didn't like soccer growing up, but I caught on and now I love it.. This sport can grow under the right conditions... There will need to be new innovative ways to market and advertise soccer here in US though..