According to Amtrak the Acela is between $69 to $171, one-way, from Boston to Philly. The regional train takes just under 6 hours. The express is an hour less.
DC to Philly is 1.5 hours and between $54 and $141.
A regular train from Philly to AC is $7.25 each way. Trip duration is 1.5 hours each way.
http://www.amtrak.com/servlet/ContentServer?pagename=Amtrak/am2Route/Vertical_Route_Page&cid=1080772074490&c=am2Route&ssid=134