You should be looking into garage and garden shed builds rather than new home construction builds because of costs. You don't need a full basement, foundation and all that. Just a slab or pillar columns for support. Use pressure treated lumber for the base and the rest should fall in line nicely for you. I've been in some nice 10x20 and 12x24 garden sheds that I've said to myself this space could make a good theater. If you sub out 8 foot walls with 10 foot even better. You could also do a double pitch roof or an extra height peak to gain some extra ceiling space in the center. Costs might be a bit more with vapor barriers, insulation, siding and roofing but then again your not comparing apples to apples it's apples to oranges at that point.
I do like the shipping container idea; it's cool. I just don't think it would look great from the exterior and it would be a shame to spend so much money and trouble trying to shoehorn a theater in an a non ideal space. I like what Tedd is doing ^ he's got good ideas.
If you end up siding and roofing this shipping container, modifying it's dimensions, and pouring cement base plates or support pillars you might as well go with more traditional construction at that point since the trouble and costs of the shipping container are approaching the cost and trouble of constructing a real structure. Saving money isn't the same as getting less because it costs less. The width is not adequate as it is, but depending on how you expand the width the idea can work.
You are right that a tight space of steel sealed up will be a bass lovers dream. You'll have no SPL problems with two Marty cubes.
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."