you can definitely do this, however, i'm a bit confused when you say:

Quote Originally Posted by kwalker View Post
Ideally, I would choose from the drop down and enter the precise square footage in one cell and the cost would automatically populate in another.
what would you like to choose in the drop down? is it the building type, then you will type in the exact sq footage? this makes a difference in implementation.

i'm guessing that you will have a drop down for building type, and a drop down for sq footage in a range (ie 1001 - 10000) and use those to give an estimated cost,
then you will use the building type and a different cell in which you enter the exact sq footage to give you an actual cost?

let me know if i'm on the right track so i can better help you out