This isn't going to come as a surprise to anyone, but the chart below is instructive nonetheless. It comes from Matt Glassman, a fellow skat fan, and it shows the cost per rider to buy up all the ad space in a DC Metro station (i.e., "station domination"). For the most part the cost runs around $2,000 per thousand riders. The two exceptions are just where you'd expect: the Pentagon station and the Capitol South station, where all the congressional staffers get off. Those staffers' eyeballs, it turns out, are worth about 4x what all the rest of our eyeballs are worth.
(I'm actually a little surprised that congressional staffers are apparently more valuable than military procurement folks, but I suppose the military suppliers have so many other outlets for their money that a Metro station is hardly worth the bother.)