Up where I cut malt barley the windrowing goes for $10-$12 per acre and we just charge a flat rate of $30per acre to combine. The barley usually yields any where from 140-150b_a. For the windrowning and combining I wouldnt do it for any less than $27 per acre. If you go with $13 per acre to combine and $.13 bonus after 20b_a that puts you right at $17.50 just to combine. Hope this helps.........