Count then multiply

I have 2 tables 1:many 

The sales table which only has the units sold...  so each row counts as a sale...  the sales table doesn't have a quantity column 

My product table has the product key which exist in the sales table as well

I want to count the number of units per product key and then multiply it by the price of each product key from my product table 

Doing the total sales by unit must be done with count as I can't use sum

I can get the count but am struggling to get the revenue with the multiply