Dennis Coates has a short piece on the DC stadium in Reason Online, which I recommend that you read. Here's the punch line:
What looks on the surface like a contribution to defray the cost of building a stadium basically amounts to the league giving a $20 million loan to the city.
In addition, MLB has refused to agree to let the Nationals keep Robert F. Kennedy Stadium as their home field two years from now, even though doing so would be cheaper by about $60 million. In other words, in the best case scenario, Mayor Williams basically agreed to hand over more than $500 million of money from a tax that soaks many D.C. businesses to a multi-million dollar operation that refuses to make the sort of cost-conscious concessions they would make if they were footing the bill themselves.