Ah, Tumbleweed houses! I've been looking at something like that for a while now.
For better and worse, they work much like RVs and trailer homes. Yes, they're quite affordable, but yes, they also depreciate in value....
To me, the biggest downside is that they use RV technology for toilets and showers. Makes it hard for it to become a primary residence I think, unless you're er very ecological, in which case, a composting toilet is also worth looking into?
Actually, I stand corrected. They also build small houses. For example, I kind of like this
Z-house, if nothing else because of the space efficiency.
The only thing is, the cost is still going to be around $100 to $200 per sq ft. which I think is fairly pricey.... You'll also have to consider where to build that, as I think some residential zones may not allow it.
Well, I guess I stand corrected again if you can build this on your own. Then you should be able to get the cost down some more. You'll still want a plumber and an electrician (unless you or someone you know is also certified in that), and you'll probably want help to lay down the foundation.
Anyways, it's all very interesting to me. Housing is perhaps the single biggest purchase most of us will make in our lifetime, so I say it's worth at least a discussion or two. Two other blogs of interest is
Tiny House Blog and maybe
Inhabitat.