How can i configure a material which has to be valuated at 3 or 4 different levels

Hi SAP Gurus,

I have a requirement like when we buy a box of materials (100 Pcs) for $ 1000, how can I configure this box of materials into 3 different types with prices. For example: 50 Large Size ($ 600), 30 Medium Size ($ 300) and 20 small size ($ 100).

It will be great if you can guide me with few steps to configure this in sap system.

Since this is my first Query in the board, please guide me if have posted this in wrong session or something like that.

Thanks and Regards,

BiTa