You see the product. You like the product. You want the product. You want to sell the product. You go to a 1/2 day boot camp to learn how much it costs. I'm not making this up.
I like flat pricing. No surprises. If you must sell something by usage package it in chunks. Which is why I use Sprint for my cell phone. If I go over my minutes, they sell me a small chunk of additional minutes at the same base price. And they flat price my land line long distance. (but not in Chicago, go figure)
You don't like surprises at the end of the month, why should your customers?