I'd suggest looking at the Bears against the woeful Detroit Lions. You want to guess who is ranked right behind the Vikings in overall defense according to PFF? It's the Bears, sitting there at eighth.

That being said, FootballOutsiders DVOA metric still has the Bears as the 28th overall defense, but that is still relying on 27% of its weight from preseason data (you can read more about the breakdown and see the numbers here).

The point here is that while I wouldn't feel comfortable saying the Bears are the eighth best defense in the league, it appears that if you want to take a chance on a cheap defense, there is merit to playing Chicago in a matchup against a Detroit team that is making a lot of mistakes.