Yes DCA is by far the most convenient as well as being the most pleasant airport. I guess that's why it's so much more expensive.

BWI is the cheapest of the bunch, not surprisingly because it's a Southwest town. The only problem is connecting into DC. THere is a metrobus that connects to the metro which will cost all of about $3.50 into MetroCenter. THere is a shuttle to the train, either Amtrak (pricey) or MARC (crowded commuter, hours not so good). But with the money saved you can get a shuttle bus no probs to DC and still be in front.
Dulles, IAD, is good for international flights and transcontinental, United and JetBlue. But it tends to be not that much cheaper, in my limited experience, than DCA. ALso transport is a little difficult as well, at least until they extend the MEtro, which I think is due in 2005/6.

Safe travelling.