For me, it has a lot to do with them being criminals. I've seen quite a few fics where Dean has an emergency credit card saved up, or they had acquaintances they could call for an emergency favor - perhaps that's th case.
More likely, though, it's another one of the show's fantasy elements, that they're appealingly poor, but it's only a problem if it's a plot thing (fantasy as in 'sweet' not as in 'dragons') - another thing it has in common with Gilmore Girls.
Bobby, though, probably has some savings, or at least some things he could sell.