I don't know how the OP feels about bus travel, but on a random weekday in July, five passengers can travel from Manhattan to D.C. for around $60 ($12/passenger) on BoltBus. Trip will take a little longer than Amtrak (about 4 1/2 hours by bus). Details at
https://www.boltbus.com
A similar service is offered by Megabus, but fares seem to be slightly higher.
https://us.megabus.com