Pricing by customer type based on order size

Folks,

I have 3 basic customer types - a dealer, a contractor and a consumer.

Dealers make a stronger business commitment to us, and we offer them better discounts and benefits to support their efforts.  Our Contractors purchase more infrequently, and therefore, receive a lower discount.

If we only had one price level per customer type, the answer would be clear. However, the final discount is based on a sliding scale and determined by order size.

Example:

[LIST]
[*]The discount scale for a dealer is either 40, 45 or 50%, and changes when the order total hits a certain threshhold.
[*]The discount scale for a contractor is either 25, 33, or 40%, and also changes based on order total threshholds.[/LIST]Simple enough? Well, there's one more wrinkle...