Due to the ecology of Calradia where it rains more then Terra's Amazon Rain Forest, there is bountiful vegetation and sheep and such, and woolly peasants to shave. Unfortunately a side-effect of all this rain is that the rocks are all worn down into tiny pebbles, and even the mighty castles are always damaged. Just look at how many ruins we play in. This naturally increases the price of rocks, especially since all of them have to be elliptically uniform in shape for proper throwing.
Also you are paying one gold for the rocks, and one gold for the woolen bag that everyone hides in their trousers.