Wingnut, how cost conscious are you?

Without checking schedules (and I should), the Route 5A bus goes from IAD into town with just a couple stops.... One in Reston, then at Rosslyn. If I were you, I would ride the bus to Rosslyn (for $2.50) then take a cab across via Georgetown and up Wisconsin Ave. The bus only runs every hour, but it will be a tad quicker- and cheaper- than the Washington Flyer/Orange Line/taxi option.

The Can from Rosslyn, VA will be metered and come to $15 tops.... the cab within DC may cost around $9. Get off at Foggy Bottom and you'll save some on cab fare (one zone less), plus save a hair on the metro fare.