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RadiantRuby As far as I know, only their expenses are covered. They may get a stipend, or something for their family members who have to accompany them. The ones making out are Dr. Now and his son who produces the show.
The benefit for the subjects is that they get top-notch bariatric surgery and follow-up care that they either couldn't afford, or couldn't find a doctor other than Dr. Now willing to operate on them. Many bariatric surgeons won't operate on people of that size. In return, they agree to be filmed bathing, eating greasy food, crying, and having other humiliating things happen to them. The point is to portray them as sick and sad as possible. This appeals to the show's audience which can look at these people and thing "well, at least my life doesn't suck that much."