My Creative Cottage/Studio
We spent about $5000 on materials and we did all the work ourselves. This is a fully insulated, electrically wired with plumbing (a sink for washing out my paint brushes) and it is heated, so this is not just a glorified shed.
My husband knows carpentry so it was not difficult for us but he did create a step by step instruction manual for the novice builder. So far there is nothing I would really do differently unless it would be to put in radiant heat in the flooring instead of using a wall heater.
Check with your local building dept, you may have restrictions or regulations on building something like this in your backyard.