PHL to BWI options?
 

Are there any transportation companies that service between PHL and BWI? Is public transportation an option? Prices? Thank you

You can use Amtrak. Take the SEPTA train from PHL to 30th Street Station (about 20 minutes). From there you can take Amtrak to the BWI station.

As a follow-on to MIG's suggestion: be aware that not ALL southbound AMTRAK trains stop at the BWI station.

Sometimes a one-way car rental winds up being the cheapest, most convenient option if you have several people doing this (though keep in mind that there are a number of tolls on I-95).

Take Amtrak from Philly to Baltimore, then take lightrail from train station directly to BWI terminal. You could take also save yourself some time and/or money to rent a car.