Freight costs to be out of standard cost for PPV calculations

Hi,

We have Freight included as a Sub-element to Material cost element. As part of standard process, when cost rollup is done, this Freight is also considered for cost rollup and gets added to unit cost. Now, for the same item when purchased, we do not want Freight to be part of Purchase Price Variance calculations. How can I avoid Freight from PPV? Please help

Ravi