Do you have a bulk supplier of mulch, compost etc, like a "fuel" company near by? That would be much more economical for a large project. I fall back or mushroom compost when I have a very large project and not enough top soil, etc of my own. It is generally weed free and has a resonably high nutrient value. It is what is left over after commercial mushrooms have been grown for a while. It's only draw back is that is you use it over and over for say 5-10 years you can get a slight salt build up, or so I have been told. Generally stay away from bagged top soil or even bulk, there are no regulations in this country to insure what you actually get except maybe in some cases in Texas.