we have a barn for our horses, my friend has a big section which she brings 6 horses into together and has recently put a lot of wood chip in it. got it free as there are some tree surgeons on the farm who have spare chippings to get rid of.
so obviously it is the cheapest option. however: you have to make sure it is horse friendly chip, not containing any plants/trees they find toxic. also it can be a bit sharp or get stuck in shoes. from a coughing point of view it still gets dry and a bit dusty, not as bad a shavings, but i think it can get more dusty than straw. depends a bit on how airy the barn is.