Around here, it seems to be done as a percentage of total sale price towards closing costs.
From our appraisal, it seems most sellers pay around 3% of the sale price towards closing costs, but our seller was ready to deliver, so we got him to agree on 6%. Win for us, as we didn't need to bring much of anything to the closing table.
As far as "closing costs" really goes, they can lump ANYTHING in there, really. It's all a numbers game.