How to sum average values

[Terrassa]

Hello,

I'm receiving invoices for PC leasing, some invoices contains some PC models and some invoices contain other models. The cost is not the same for all the PCs with same model, so I'm working with average cost per model based on the last invoice received for each model. I need to calculate the total monthly cost for all the PCs.

The grand total should be the sum of all the values in "Average monthly cost based on last invoice", but it's actually calculating the total cost of only the PCs received on the last invoice (25 PCs received on Sept. invoice), using the average cost of all of them regardless of the model.

How can I calculate the total sum of the last column?

[ryan_mayu]

maybe you can try 

sumx(values([pc model],[the meausure of last column])

[Terrassa]

With the formula you indicated I'm getting the following error:

Too many arguments were passed to the VALUES function. The maximum argument count for the function is 1.