For ease of construction and cost consider the gothic arch. It can either be laminated or use inexpensive lumber.
It can be made into quite a nice little cabin for not much money. If you go with a tamped earth floor or other natural foundation it can be very inexpensive.
Here are some examples, but it can be done as two story as well. There is a gentleman in Russia that has plans for a 200 sq ft building that I have lost in my bookmarks. If I find it I will add it to the thread.