K.I.S.S - Keep It Simple Stupid
Use this method when pricing your design services. Don't charge what everyone else is charging and you will get the job, it's as simple as that.
Almost everyone who comes knocking at your door for your services does not have a fat pocketbook to give you a large portion of. Netizens and non-netizens alike are becoming more aware of what's fair and what's not in the web design world and $50+/hr isn't fair, especially if the banner takes 30-45 minutes to build.
If you price your services on a case by case basis then you will see that the end result is better. Even if you charge less, you will get more jobs, which pays off in the long run.
Don't charge more than you would pay for a banner ad. It shouldn't take that long to create (if you work efficiently) and should be relatively easy (if you are good at what you do). Given, the banner may be more complex than normal so price accordingly.
Tip: If the price is right, they will buy, if the price is not, they move on...the lowest price is just a click away on the Internet.