How much would sharing the cost of hay and shavings cost?

You really just need to ask the stable owner. This number could vary greatly depending on how many people you're splitting the cost with, how many hours your horse spends indoors vs. outdoors, and how much good grazing is available. Different types of bedding are sold for different prices and the cost of hay can vary greatly depending on where you live.

You really need to ask the owner of the barn. All supplies cost different prices in different areas, and even between suppliers. Also depends on how much you might need. A pony would cost less to feed and bed than say an 18hh hunter! He might have a good supplier who gives a good price if items are bought in bulk, but do check the quality.