There are several Comfort Inns in Philadelphia, and they run about $98. I checked the historic area and another area, and they were both about the same. I'm sure you'd get a lot better price if you stayed in a smaller town outside of Philadelphia and took the train in... that's what we did when we went to Boston. We stayed in Salem, and it was half the price. Here's Comfort Inn's website:

http://www.comfortinn.com/

Also, you might try http://www.priceline.com A lot of people have gotten incredible deals on hotels in big cities.

Sandy Illinois