I first would like to say that I think this is a great idea and would love to see you make it happen.
However here are some things to consider before you commit.
What part of california do you live in? There are state, county, and municipal codes to; consider some of which vary by loaction. While it may be obvious that its a temporary structure to you, the govt may not feel the same way.... and even some temporary structures require permits.
Temporary or permanent you will need a foundation, the reasons why should be obvious. The type of which will vary depending on what part of cali you live in. Since you most likely live in a residential or rural area its not likely that geotechnical engineering surveys/analysis have been done that would accomodate an easy process for design of the foundation. If you do live in a residential zone the data may be there, but this is very different than a house and some work would need to be done to design the foundation.
Permits and design/construction of the foundation could be costly, though not necessarily a deal breaker if you are commited to it. I think you are over simplifing the cost as well. It will be much more to deliver one of these containers to a private residence than it does to a repeat contract customer. The fuel alone would likely exceed $250, and the shipping company is not going to unload it for you, you are looking at needing a large forklift with large rigging, or a small crane, either of which will cost you at least $1000 just to move on site.
Not trying to rain on your parade, just want to get you started off in the right direction before you get in too deep.