If you raise preflop to 4x, get called by BB you have 8.5 BB in pot, you bet 6 on flop, he check raises you, you let the hand go. So you've risked 6bb to pick up 8.

So if you do this scenario 2 times and he folds once you've lost six BB and acquired 8.5, therefore it is +2.5bb to do it half the time. If you have someone playing back at you more than 50% of the time on your raises you've got a gold mine. Most players are going to lay this hand down far more than 50% of the time, therefore its a much larger return than the conservative example given. Hell even if he's playing back at you half the time you still have a gold mine. For most players this is going to be a 10BB per three deal, if not more the tighter the player.