If you have "land", then the ubiquitous "single wide mobile home" is usually a better choice than the "same thing in a shipping container".
By the time you turn an ISO into a lodging structure (as many firms do, check the Sea Box site for great ISO container pron) they cost more than a single-wide.
Horseshit. They cost more if you buy them completed, they cost more if you buy a kit in many cases (though not all) but they do NOT cost more to put up, nor even nearly as much. You can get a 20' container delivered for under two grand even in bumfuck where I live. Granted, it's Bumfuck CA, so I'm relatively near a port, but it's three hours by truck from where the containers are to where I am, one of them on twisty and hilly roads. If you bought them in bulk you could likely get 40' containers for a thousand a piece. Empty containers are stacked up so high at some ports that they're a nuisance.
They don't necessarily save much if anything in construction costs if hiring out the work, but you wouldn't know that from the many people pimping them.
It depends on how fancy you're planning to get.
I just spend the day welding a splice strip between my roofs
I presume you don't live in quake country, or it would be preferable to leave them separated.
and after coating the roofs (about a grand in materials)
If you were building apartments for the homeless, you'd stack the containers. You'd put walkways and stairs at one end with an entry door, and you'd put a window with a fire escape ladder at the other end to meet code. You'd put a greenhouse on the top of the structure, eliminating most of the roofing requirements entirely. Or you'd build a green roof there, which consists mostly of laying down a layer or two of fancy plastic. Either way you'd go up three stories or so, taking advantage of the natural properties of shipping containers, and your roofing costs would be reduced that many times. You'd trade them for the cost of building walkways, but those could also be built from scrapped containers...