Difference in Price Type Totals

Hey there, 

I am trying to create a widget that shows the total amount of currently unredeemed vouchers. My current widgets are Original Paid Amount with a filter for the "Purchase" price type. and another for the "redeemed" price type. When I try to make an equation subtracting the two, it gives me an error. 

Formula;   (SUM(Original Paid),(Price Type - Purchase)) - (SUM(Original Paid),(Price Type - Redeemed)) 

Error definition in functions (SUM): Expected numeric expression, found "set" 

Any ideas on how to accomplish this, or why this equation is not working? 


Thanks all!