What a timely question. I was just flying out of BWI last week and stopped to buy a bottled water at the coffee place in Terminal D. When the cashier told me it was a mere $1.04 (less than I would pay almost anywhere "outside"), I was so shocked I almost asked him if he had rung up the right item!

So, for bottled water, the answer is definitely, BWI, Terminal D!

As far as food...

Anywhere with a Potbelly, they're always $3.99/sandwich whether they're in an airport or not. So that would include IAD, MDW, and DCA (as of very soon?)

The Greek place at MDW is also good and reasonably priced.

And not that most of us will ever fly from it, but TRI has a cute little locally owned sandwich shop next to the Northwest gate (one of, I think, four gates in the facility) with reasonable prices and friendly service.