With a new fare increase metro has put in what I think amounts to a tourist tax.
There are two ways to pay for a metro ride a paper fare card or a Smarttrip card. The paper cards can be purchased from vending machines in every station. Smarttrip cards can be purchased at vending windows in just a few stations and CVS. The Smarttrip card cost $5 plus whatever fare you put on them, while paper fare cards have no upfront cost before the fare.
With the new fare increase rides with a paper fare card cost 25 cents more than a Smarttrip. So in less you go on 20 metro rides, it's cheaper to use paper cards. On average though tourist who only use the metro a few times will pay more per trip than a regular native rider.
Even better for DC folks Smarttrip cards will be reduced to 2.50 at the end of August.
Final thought the fare increase is because the metro system is not earning enough money. Since metro has a monopoly on mass transit, this means it could gain even more profit by raising prices even further, but it is not a profit maximizer.