Struggling...and I'm sure it's basic...

Ok... So I have the quantity, and then the price, plus the surcharge = amount/sum. Here's what I need help with:

When there is no quantity entered, how do I get the sum cell to be a "-" (dash), but then autosum the correct amount when the quantity is entered? 

Ex. 

Quantity 1 x $200 + 15 = 215  ------  and then Quantity 2 x 200 + 15 + 430

without having to change the formula every time? Does that make sense? 

But when the quantity is not entered, the SUM cell is just a dash until the quantity is entered and then it knows what to multiply? (NOTE: The price + surcharge are always the same amount and entered already. Only the quantity changes, and my biggest issue is getting the SUM cell to dash when the quantity is zeroed out).