United has round-trip flights from Austin (AUS) to Baltimore / Washington D.C. (BWI) for $148 with a connection. $74 each-way. Flights February to July, mostly on Tuesdays, Wednesdays, and Saturdays. Sparse availability in June. Midweek only in July. Origin/Destination reversible.
Note: Fare is “Regular Economy” not “Basic”. Carry-on bag, and seat selection are allowed for free.
Note: Spirit and Frontier have flights for about $110. You should avoid those unless you plan on not bringing any luggage, or selecting your seat, or having a drink. We don’t consider Spirit and Frontier to have competitive flights unless you can travel without paying their +$60 bag fee.
Read More Cheap Flights: Austin to/from Baltimore / Washington D.C. $148 r/t – United