Comment 3 for bug 646350

I don't see a way to do this.
We do average of prices instead of sum(price*qty)/sum(qty). As it depends on the way you Group By, I don't see a way to do that.
Even if we do Total Price / Total Qty, as it's computed on the detailed level, it will not work.

OpeneRP supports only two attributes on a field aggregation (sum, average). We can not do complex formulae for group_by. We can do an average but not a ponderated average.