You may want to re-arrange that to be for your first full day in town, because those tours are an excellent way to "get your bearings" once you arrive. After that, start buying
MetroRail one-day passes. This will be vastly more convenient for you, especially after a long day of walking around The Mall and such. The Metro is quick, convenient, clean, safe, and quiet. My favorite subway system in the world (and I've been on
a lot of subways).
I would guess well over $100 each way from central DC. It's about 40 miles away. Far better way to do it: Rent a car for one day in the city itself (not at the airport). If you do that on a weekend day, it should be plenty cheap. A quick-and-dirty pricing of a car picked up at Union Station on June 4 for a 24-hour rental is $27 plus tax for a total of $33.68. That gets you the car without your having to pay $40/day to park it in the District, and it would afford you the possibility to go drive around some of the pretty Maryland countryside around there, too
Best money-saving tip: stay in Arlington or Pentagon City or Crystal City or Alexandria and take the Metro into town. You get
much better deals on hotels across the Potomac and many of the hotels there run shuttles to the Metro station(s) as well as Reagan airport. You can get something like an Embassy Suites or a Residence Inn that has a fold-out sofa for the girls and feeds you breakfast in the morning and whatnot for ~ $250/night or better, and only have to pay for one room that way without being stacked up like cordwood like you would be in the District.