It isn't any cheaper to get high PE orbit capture than Low, if you make maneuver node to lets say mun periapsis when capturin, make it with both approaches, you will see that with same amount of dV you get circular orbit around mun, other is just higher than another. So to get cheapest you must aim low because you can then do high eccentricity orbit which costs less dV, and it is not possible to do when high orbit, with same amount of dV used.