310875 Joomla Price calculator

Hi I need a joomla component or module to calculate the advertising rates charged for 11 different directories - giving a discount for volume - this wants to be easy to implement and easy for the end user to use - can be a series of check boxes or drop downs or whatever works best - please let me have your ideas as to how it could work and the costs involved. Feel free to ask any questions Many thanks Paul Pablodesign