Formula for Encumbrances

I need help with a formula for a summary report that's a BVA. I need an encumbered field, which is basically all the contracts related to a Project. Some Projects only have Activities though, no contracts. Right now the formula works for the Contracts. The rest should be something like- If no contracts, then sum expenses.
Here's the formula I have:
Sum([Total Contract $ (after Amendments)])

------------------------------
Bethany Hudson
Program Administrator
Mobile Bay National Estuary Program
Mobile AL
2514598870
------------------------------