To prep a job like that you are looking at 4-6" of well compacted 3/4 process and then the stone set on top in stonedust or sand. Some Masons mix some portland in the sand to hold it tight. I like stonedust for ease of installation and ease of finding material for future maintenance.
If you were going to install them in cement you would want a 4" slab than set the stone on top. Though most masons will prep the base with clear gravel or process and set the stone right on top of that. I'd be careful if your installation is near a pool, there may be some settling where material was excavated.
Pricewise, depends where you live and how bad the mason wants the work. Though I hear those guys are in pretty high demand right now.
Did you get any prices yet, I'm curious what the going rate is in your area?