And what is done for the foundation generally depends on what people have to work with. Building a wooden floor is easy, plenty of people do that - as I said, its easy, its moderate expense, and it holds up for a few years. On uneven soil, you can build it as a deck, raise the thing off the ground some, deal with runoff at the same time. Only problems are rot and bugs. Figure that deck will set you back another couple hundred and a day's build time.
Others use concrete - frame it out, figure the cheapest way to fill it - 8x10x4" deep is about 1 cu yd, roughly two tons, or about 50x 80# bags. Lots of labor and weight to deal with, but too small for a typical concrete pour to be worthwhile - unless you are pouring other concrete at the same time.
Still others have an existing patio.
Or you can just use earth augurs and tie downs, set a metal skirt *(like some old chain link fence)* and cover with a deep layer of mulch.
