I did my share of working in dirt and on outdoor slabs when I lived in michigan... I now live on a farm with my folks in Northern Wisconsin with a 40x48 heated slab shop.... I don't miss the dirt days.
Look into seeing what some low-grade concrete would cost you.. 4" of 4-bag (6 bag is standard concrete.) mix is fairly cheap for a 20x20 or so area and would be a great thing to have. You'll never regret it..
