Return sum of values in column when matching and also when Row matches input

In the attached sheet, I have many products listed in columns, and the columns repeat with the following (Purc. #, Cost, Cust. Inv )

I am needing help making a formula that will, sum the total cost for the a specified Purch. # and specified product id.

I have tried all sorts of ways to do this but a search based on two inputs is too difficult for myself to works out. I thought it would be easy at first, but no luck after trying for hours.

I hope someone has an idea how this can be solved and Iam fascinated with what the solution is.

Thanks
Dave