Totally Lost -- Please Help

I am looking thru SQL Books, browsing the forum, but still unclear of the best method to perform the following calculation. I am really struggling with this and any helpful hints would be greatly appreciated.

Situation
Table 1 = customer information. Contains customer #, name, address, etc.
Table 2 = default price list. Contains item #, item desc, price
Table 3 = Allowed discounts based on customer types. Hospital gets $1 off, Doctors get 25 cents off, individuals get no discount.

Further discounts are given for multiple purchases. The 1st item is 100%, 2nd item is 75% and 3rd and subsequent items get 50% off purchase that day.


Here is a basic query result set of customer number, price, item ordered, item description and customer type.

Cust Price ItemNo Item Type
1 $5.50 23432 Moisturizer Hospital
2 $5.55 23453 Moisturizer 2 Indvidual
2 $29.00 23243 Lotion Pkg Indvidual
3 $29.00 23243 Lotion Pkg Doctor
3 $21.25 23232 Gloves Doctor
3 $3.00 23235 Drape Doctor
3 $2.00 27821 Lube Doctor


Calculation

Customer 1 ordered 1 item @ 5.50, but because they are a hospital they get a $1.00 discount. They only ordered 1 item so they pay 100% of the discounted price.

(5.50 - 1.00) * 100% = 4.50

Customer 2 ordered 2 items @ 5.55 and 29.00 respectively. They get no discount because they are an individual client, but do qualify for the multiple item discount of 25% for the second item.

29.00 + 5.55 * 75% = $33.16

Customer 3 ordered 4 items @ 29.00, 21.25, 2.00 and 3.00 respectively. They get a 25 cent discount off each item for being a doctor. They also get a multiple item discount of 25% off the second item and and 50% off items 3 and 4.
The math ideally will look like this

(29.00 - .25)*100% + (21.25 - .25)*75% + (3.00-.25)*50% + (2.00 - .25) * 50% = $47

Problem
For customer 2 if someone entered the lower valued item first the math would look like this

5.55 + 29.00 * 75% = $27.30 (loss of revenue)

Possible Solution
The prices need to be sorted in descending order to maximize revenue prior to discounts being taken for a given day.
Should I use a "rank by clause" statement?

Then I need to multiply the first item by 100%, the 2nd by 75% and the remaining items by 50% to get a grand total of money due. Any pointers would be greatly appreciated.
I am not at all sure how to accomplish this.