I have a LOT of stuff, mostly tools and materials but I am also sentimental and love history so I have tons. Literally truckloads

. I have been most happy in a small/tiny/micro home. When I see so many people get rid of stuff that they may need and going on a minimalist rampage it make me shake my head. The key, as mentioned above is to keep the storage outside of your actual living space. This is better in many ways, as mentioned only heat/cool what needs to be, things can be stored at a cooler temperature than in a living space, less risk of losing everything, etc and yet everything is there when you need it.
I have lived in a "bunkhouse" that was something like 8x16, a travel trailer, and a 24x24 cabin and enjoyed everyone. My favorite is by far the cabin of course but realistically it is much more room than one person needs. Many families around here had a "summer house" that was much smaller and was used in warmer weather when people are outside most of the time anyways, this worked good for storage because you could shift stuff from one to the other depending on which was used.
The cost/square foot doesn't really matter imo, that's like saying "how much is that car worth/pound". Most people building these are spending a tiny fraction of what a "normal" house costs, that is the significant factor. I do have concerns with many of them though, people don't typically anchor them, they often have a low mass (thermal and structural), and many are putting looks at a higher priority than functionality or durability.
gabbyj310, I really like that idea, if you have a hillside to built it into it will probably be easier. Other than a backhoe not much in the way of special equipment would be required. Of course it could be re-purposed at any time as an excellent storage space or even framed in to make a living/working space on it's own.