Item base price (Order level)

Hi,

I am having query related to pricing.

suppose an order has 3 items - I1, i2  and I3. We want to be able to sum up quanity off all the three item and use that quantity as a Tier Break to get price for each item.

Example -- Order qty for I1 is 2, i2 is 5. They sum to 7. Our base price should pick the base price of quantity 7 for i1 and quantity 7 for i2.

Can someone suggest , how can i implement it ?

Thanks,

Vishal