14298 Paypal - Calculation Script

I'm looking for a method to calcualate the cost of membership for users of my membership site. The price for membership is decided upon the amount of counties they select. There are currenlty 15 counties available. The user would select 1 to 15 counties. The County Listing is as follows: Apache Cochise Coconino Gila Graham Greenlee La Paz Maricopa Mohave Navajo Pima Pinal Santa Cruz Yavapai Yuma Depending on how many check boxes (or some other method) that are selected will dictate the cost they need to pay (via paypal). 1-3 Counties $5.00 each 4-6 Counties $4.00 each 7-10 Counties $3.00 each 11-15 Counties $2.00 each After the person has selected the amount of counties they will want to be listed in, they pay via paypal with the total calcuated by the script I need created. I would like a few other fields, including the counties selected to be provided in the comments section of the paypal reciept, or email that is sent after processing.