2 ways you can do this:
Solution 1:
When adding purchases simply make purchase price 0 for the free items.
Solution 2:
Find the average price of the item and use that for unit price.
for example, if you purchase 10 quantity for a price of $2 each, 10X2 = $20
and get 2 quantity free, then the average cost of each will become 20/12 = $1.66