I've always gotten boulders for free from farmers. They're pretty plentiful where I live. Of course, moving them isn't free. In places where they are not plentiful, I've heard that they can be expensive, since someone has to haul them from somewhere else. You should probably ask around locally.

I can tell you that if you have to buy them it can stack up pretty quick. We are fortunate to live in an area that we can get one or two here and there as we need them, or can move them. We plan to buy some for our yard once the main projects are out of the way. I guess price also depends on your location, but here's a wholesale stone company we are going to buy from.

Here is a link that might be useful: Alabama Wholesale Stone Company

Here in Az they start at $160 / Ton