We have done several of these type of projects at various properties this past year. Usually they are due to islands that keep having problems with tire ruts due to high traffic locations at commercial props. Our cost is btwn $o.o5-0.06 per lb for the stone. We use what is refered to as a "Pond Mix". It has stones that are 1-2' in diameter, down to 4" stones. We typically will line the bottom of the bed with fabric, to help with the dirt and and weeds. Then we stagger the placement of the larger stones, fill in with the remaining stones, and then lightly scatter 1-3" stones to fill in the little cracks and crevices. We find that the "Pond Mix" will spread about 80sq.ft. on average. (That is about a ton) Every market is different as far as their availability of stone, but good luck. I will be on one of our recent jobs later today and will take a pic and post it later to show you what our finished product looks like.