Mapping Multiple Dependencies

Mapping Multiple Dependencies

What I am trying to do is somehow have a field that calculates the product price based on some criteria that the user enters in our web-to-leads form.  Let me explain exactly what I am trying to do:  our company sells flash drives based on quantity and capacity.  So what I mean is that We sell flash drives in quantities of 25-50,51-100,101-250,etc.... and capacities are along the lines of 64 mg, 128 mb, 256 mb, etc.... the prices are like for a 64 mb capacity with quantity of 25-50 around $10 or a 128 mb capacity with quantity 51-100 is $8.  Quantity and Capacity are picklists on our site that the user selects what they need.  I need to use this information to calculate the price of what the user wants and store it in a field in crm.  how can i map such a multiple dependency?  Please help.  Thank you.