Your project will require 8 cubic yards of concrete at 4" depth. Concrete will generally about $100/cu yd depending on distance to the batch plant and local conditions. With base and rebar you are looking at about $1100 (excluding labor). To build a wood deck using appropriate sized treated lumber will cost considerably more. Wood floors have been used in garages and shop buildings but today the price of reinforced concrete is much less than wood and there is a great deal less labor and performance is better for concrete. If you want to spread the cost out, consider pouring just your foundation and stem walls and laying the gravel where the slab will eventually go. You can add the slab at any time.