Depends on what you'll be paying for. If you want to build with only#1 lumber, use real 2x4's instead of 2x3 studs, space the studs at 18 inches instead of 24, hang double pane windows, insulate, use 3/4 inch plywood (for floors/walls/roof instead of particle board), put in a nicer flooring, have it on a real foundation vs skids and pay someone for the labor. I can see a custom shed being 5K start to finish, if not more if you lived in the greater Seattle area.
But if you are doing it on your town labor, can salvage materials, don't mind making things fit here and there, you can do it for way less.
But if you are doing it on your town labor, can salvage materials, don't mind making things fit here and there, you can do it for way less.