It will cost you 2-6 dollars one way, and take you about 50-65 mins take the Red line to Metro center and get on a train to Ashburn.
The walk from the station to the terminal is 5 minutes in a warm tunnel.
2 dollars right now on the weekend, probably about 6 during the rush on a weekday. I checked out the trip the weekend the silver line opened as I fly international pretty often. The numbers Google maps gave agreed with my own timing of the route on the day I did it.
You’d want to take the red line metro from Union Station and transfer to the silver line at Metro Center. DC metro is pretty straightforward, lots of signs around, and the Dulles stop drops you right into the airport. Lyft/Uber will likely be quicker but I personally wouldn’t take a car to Dulles for more than $35. Depending on the time of day, you’re not saving much time by driving.
LAAFB_throwaway t1_iy1ep8c wrote
https://www.wmata.com/schedules/trip-planner/
Metro red line to metro center then silver line to Dulles