Calculated field in DAX: divide a column number by another

I created a Data Model using Power Pivot in Excel. It looks like this:

Now I need a calculated field to calculate the % margin. That is given by:

MargemProduto/PreçoProduto

In DAX the expression I'm using is:

=averagex('Vendas';Vendas[MargemProduto]/Vendas[PreçoProduto])

Although this seems ok it doesn't seem to make sense to calculate an average since what I'm trying to do is to calculate a fraction.

What DAX expression should I be using instead?