Just remember, most of what you charge should be to cover food, hay, and shavings. Also, make sure people understand that only the feed is included. They will need to pay for additional feed supplements.
When prices in hay, shavings and feed drop, normally board drops slightly. Same for price increases.
To determine your price, you just need to take into account some numbers that you can find out through local research. Like I mentioned, find out the prices on feed, hay, and shavings, and account for your labor. Then tack on a bit for the use of the facilities. Having horses around always puts wear and tear on your buildings and fencing. Add it all up and you will basically have your fee.
I pay $360/month for a fairly private place with most of the features you have listed. The owners are very fair though, and I get a pretty good deal for the quality of their feed and hay, not to mention the care my horse gets. At other barns in the area, I could be paying $400-425/month for the same things.
Just check around with the other boarding barns and look into the prices and qualities of the feed, hay, and shavings in your area.