Hello,
I'm from Colorado and here I pay $6.00 per bag, for the 3.5 cu ft size, except when they put them on sale for $5.50 (Yippee!). I go through one bag about every three months. I have the same size coop that you have and I don't have poop boards, I just pick the piles up off the top of the shavings with a kitty litter scoop or a doggie pottie pick up bag. I gave up the boards in favor of more room for my six birds.
I had mine very deep this winter when it was so cold but haven't really backed off much. The shavings are about six inches deep and I clean every other day and fluff up the shavings. I keep them deep because I read somewhere that leg injuries can happen if the girls land too hard as they fly down from the roost. So far, no leg injuries.
There are two types of shaved bedding; wood chips and pine shavings. I use the shavings, easier to clean and they are thin enough so that they compost more easily. Some of the wood chips are fairly hefty.
Good luck, Mary