Interesting?! He was implying this was just additional material costs (plus transport) but not that he was adding labour. Perhaps that is why he has not yet provided the receipts I have asked for...
You can get decorative stone chippings for less than what he is asking, if it is indeed just materials. I'd say call a local landscaper, and ask for prices for topsoil - ask for their best one with enhancer, so as to give your guy a maximum price per tonne available!