If it's anything like the full-time construction volunteers I know, they are total borg zombies. The couples I know either rent out their houses to other JWs (who might pick up their window cleaning business or something like that) while away on projects, or just sell up altogether and live out of suitcases. Some who have been in it for a while and get the "full-time status" get a modest stipend, but not much - certainly no more than a bethelite.
It's all very short-sighted though - keeping themselves afloat depends on the WTBTS giving them continuous assignment after assignment, but like many full-time volunteers once they have outlived their usefulness and no longer needed there is nothing to stop the borg dumping them out on the street at a moments notice.
They will then be in a position possibly without a house and unable to buy one because house prices have gone up in the mean time, while what little savings they had were sat doing nothing in the bank, and likely no pension either. They will also find themselves too old to start having children.
You think you're missing something but you're really not - these people are so deluded to devote their entire livelyhood to the WTBTS, even more than regular dubs. You'd have to be crazy, and they are.