DCA and BWI are not co-terminals. The fares vary significantly, and sometimes DCA can even be cheaper. DCA/IAD are more related.
DCA and BWI are considered different airports. They may show on some websites as alternative airport, but that does not mean they are interchangeable.
For the past couple of years I have not needed to fly out of BWI - as the DCA fares have been nearly as good or in some cases better than BWI. Although right now with Southwest fare sales, BWI is cheaper.