Subway advertises a lot for opening your own franchise. I would check their website as I'm sure they have information on the start up costs. Hang on...i'll even do it.
This link will help you out big time...
They say the franchising fee is $15,000.
The capital required to actually purchase a location and the proper equipment varies, but they say it can be as low as $78,600.
That seems pretty damn low...I'm guessing over $100,000 at least on average but I'm sure you could e-mail them and tell them you're doing a school project and need questions answered...companies are usually open to help in that situation