Peter Pan is as low as $8 if you buy ahead. Greyhound is more but can be less than Amtrak- both leave from near Downtown (not like Megabus that doesn't really serve Baltimore but claims to). But for my money I'd take Amtrak.

You might be able to find cheaper busses, but Amtrak is fast, reliable, and not sketchy.

I used to go to Philly once or twice a month for work.

definitely Amtrak! not sure if its the cheapest compared to bus prices, but I've done it over ten times to Philly! I book with their app! When I book early it's always super cheap but honestly when I head to Philly or NYC I take the 6 AM train and it's always super cheap haha. Never had a problem, just sleep on the train! Being from Dallas, I'm always super impressed and thankful for this kind of public transportation/infrastructure.

Train will be a lot more comfortable. Busses are cheaper sometimes but way too crowded.

(Technically speaking, walking is cheapest, but I wouldn’t recommend it…😅)

Look at Mr. Fancy Pants over here with his shoe leather budget. I prefer to wait for plate tectonic shifts to get me from place to place. Slower than walking but the price is right.

Absolute cheapest: bus

Amtrak Northeast Regional is well worth the extra $10-15 over the bus ticket though, in terms of both speed and reliability.

MARC to Perryville > Cecil Transit to Newark > SEPTA to Philly

Megabus

As everyone else is saying: if you can stomach the cost, go with Amtrak. It's literally one hour door to door. A bus is half the cost but twice as long

Flix bus is an option as well. $15.99 Not sure how convenient this option is to your destination but at least it's something.

I've used it to go to DC from Baltimore and vice versa but never anywhere else. I do know that lots of people use it to go from NYC to DC and back and that's definitely a lot longer than from Baltimore and Philadelphia.

Amtrak never checks tickets anyway.