Custom Opportunity Field times Sales Price

i want to creat a field called PC Amt (PC_Amt__c) = PC_QTY__c * Sales Price (sales price from the products)

Problem is when i try to create a new formula field in opportunity and i select PC QTY as the field and then the "*" i have no way of selecting Sales Price from the fields drop down menu.

Does anyone know how i can do this?