If you have a 4x4 garden (16 cubic feet) but it is only 6" deep it will need 8 cubic feet of mix to fill. So you would need just under 3 cubic feet each compost, peat, vermiculite.

A $10 bale of compressed peat moss should expand to 7.5 feet I think...so you would use half a bale for one bed ($5)

Vermiculite can be found at building supply stores and pool stores for as little as $12-$13 for 4 cubic feet. You would use 3/4 of the bag for one bed, so $9.

For just one bed you'll probably be buying bagged compost. The good stuff can run $4 -$8 per cubic foot. Figure $6 per foot on average, and you need 3 feet, so $18

The cost to fill one bed with bagged stuff is somewhere in the $30-$35 range. If you are filling them on a large scale you can cut down on the cost by hauling truckloads. Or, if you have access to your own or a neighbor's compost, you can cut down on the cost considerably.

Jo