I have such a retreat in the boonies about five hours from home. Primarily go their hunting and fishing, but sometimes just to loaf too.
Rather than a modular, I built a post frame building, all metal sides and roof so very little maintenance. About 2/3 of it is finished out very nicely into a 2 bedroom living quarters with full kitchen, bath, bedrooms, great room, wood stove, heat, AC, etc. The other 1/3 is garage space. It also has a nice covered porch.
This location happened to have municipal water at the road, so no well needed, but I did have to get a new electric feed and septic system. All told, I've got right at $100k in the place, not counting the land cost.