Jail dummies are impressionable, naive, usually young people who are just entering the world of low end crime. The jail dummies have no clue what the profit from their crime is actually worth, because they're new and oblivious to the profit margins and believe the criminal hiring them is simply giving them a fair share.
In truth, the crime itself is usually simply but fairly high risk, which is why a smarter criminal will outsource the job to jail dummies, so the dummies go to jail while the career criminal gets to deny the whole thing.
Don't worry, if we get caught, I can deny the whole thing and blame it on my jail dummies.