what is the cheapest type of battery that i can replace my old battery with in a 2001 ford expedition?

Walmart, Autozone, or any place the does rebuilt batteries. I am warning you though you get what you pay for when it comes to batteries.

Johnson controls makes batteries for over 52 different companies world wide. There premium batteries such as an autocraft gold from advance auto parts is made with there power frame technology Using new lead for the plates inside. While the plastic case is made from recycled plastic, there power frame lead plates are new and stamped not poured into a mold. When it comes to new batteries you do get what you pay for.

Yup thats pretty much what I said. Walmart and Autozone batteries (Neverstart and Neverlast) are junk I replace more of those than any other batteries. They're cheap and do not last half as long as an Interstate, Napa Gold or even Die Hard.