Alix, we do as you do.  We averaged the cost and began offering a flat rate some time ago.  You win some and you lose some, but in the end it's pretty much a loss .  We reserve the right to bill additional in the case of excessive copy costs, but I don't think we have ever had to invoke that.  We offer a choice between flat rate ad ala carte.  I prefer the flat rate because it keeps billing simpler and we don't spend our days counting pages.