Well, I to am in that same thought of which would be the best bang for the buck. Although I am not looking into pre-fab due to the extra costs involved. Plus allowing extra eyes around my site. (just my take on it) Plus I do have some building skills with concrete, metal, and wood structures. So labor costs are contained to sweat equity.
I personally would not put to much consideration into a shipping container due to the cost of the unit and then to tie up even more funds to adapt it to work for my needs. Average cost of 1500.00 to 2500.00 plus structural improvements to make it viable due to the weight loads involved. Which the structures frame is the really the backbone and the rest is purely sheet metal. Not to mention water proofing 6 sides properly with out heavy equipment to lift it.
Concrete, when done properly would not be all that cheap. It would be met with alot of bracing, forming, and alot of sweat hours. But would be one mean Hombre if the time arose.
I however am seriously considering 10 foot metal Culvert as my means. Huge weight toleration, combined with no form work.... (which that really makes me happy lol). Welded end caps with considerations for reinforcement seems doable. and all of the typical considerations for plumbing, air, electrical and multiple entry/exit needs are met. Water proofing, drainage and then septic and backfill needs. The only thought is would this type of construction eventually rise even though on a hill side with lots of drainage?
I am going to construct this in the southern half of Alabama, on a hill side. If anyone has any experience with this type of culvert construction and wants to chime in with any tips or considerations I would greatly appreciate it. Also, if you have constructed other types of structures here in the South and want to share any info I should be thinking of would be appreciated as well.