Hello there, can anyone suggest the best way to do this?..

I have set up 3 price attributes for retail, 1 bottle, half case, full case. I want my trade customers (who will need to be manually approved) to be able to select a trade price for the full case (which is a flat £15 off the full case retail price). I don't want anyone else to see the trade prices. what's the best way of doing this?