Why are flights out of Boston cheaper? [duplicate]

Law of supply.

Probably more people fly from Boston to Europe than from Baltimore (and maybe from Washington, I did not look it up)

Also, you add more distance and more distance costs more fuel and more fuel costs money.

(from googling flight distance from Boston to Paris)

Flight time from Boston to Paris is 6 hours 30 minutes

Distance from Boston to Paris is approximately 5540 kilometers.

(from googling flight distance from Baltimore to Paris)

Flight time from Baltimore to Paris is 9 hours 11 minutes

Distance from Baltimore to Paris is approximately 6140 kilometers.