Well, it's obviously not just the rent. There are the economic benefits of the redevelopment of the area. Of course, there is a huge disagreement over how much of that is attributable to the Nationals, and how much is simply due to the fact that this was an enormous, largely undeveloped area with metro access on the right side of the river in a growing and wealthy city. I think much of the development would have happened anyways, although it was probably sped up with the Nationals bringing 35,000 people to the area 81 times a year.
But it's impossible to ignore the fact that there are new businesses, 3,000 new residents, meter fees, parking lot receipts, etc. that are to some degree driven by the stadium being there instead of Ashburn.