I tip 15% for pizza delivery, more if it's markedly fast. He's a waiter, but instead of walking it to you, he is also scratching up his car with that sign, burning his gas, wear and tear on the car, etc.
No disrespect, but if I feel that I don't have the money to justify paying for the pizza AND the delivery (but am just not going to go pick it up myself), then I will just buy a frozen pizza and doctor it up at home. I kinda feel that if I don't have money for a tip, I ought not be paying for the convenience food to begin with. We usually do pizza delivery about once every 2-3 months (for a babysitter, or a tv sports night where we are busy with guests, etc).