Originally Posted by Vidaloco
If you are talking about the place where the horses just hang out I would suggest limestone screening. We put it down in our dry lot and a big mound of it where we put the round bale feeder and it has been a God send. I spent last spring losing my boots in the mud. This year its been nice and firm I can go out in my garden clogs and not have to worry about sinking in the mud. The horses aren't standing around in slushy mud when they are at the bale either. I think its conciderably cheaper than sand too. If your talking about an arena or round pen training area I would go with the sand. The screenings harden to almost concrete once its packed down. Check with a local dirt hauler in your area to see whats available and check prices.
I'd like to add that the limestone screenings really helped prevent thrush in Vida's herd, over the wet winter when a lot of horses hooves were drowning in mud!