Nicky,
If it's 30 to take the bus from AC-DC, I HIGHLY recommend taking Amtrak for 20 bucks more (50 total). It's much more comfortable and MUCH more convenient (leaves frequently, gets you to Union Station which is better). Go to
www.njtransit.com for local train times from AC-Philadelphia (leaves every hour or so). Trains from Philadelphia 30th Street Station-DC leave every half hour or so (unreserved trains are cheapest). Let me know if you have other questions.
Jeff