Do they own the plane, or do they rent it or something?
I know Jim Bob is likely more wealthy than we realize, but he has a lot of dependent people who don't appear to be supporting themselves to pay for. There are 6 minor kids in the house, plus Jana, and another few probably still living there. Then there are the adults who we can be pretty sure aren't supporting themselves, like Joe and Kendra and their 3 kids. And Josh's family of 9, plus his legal expenses.