Hi Nathaniel: I know it's tempting to tell your customers exactly what you think, but I would just let it roll off you. At this point, it sounds like they just want to complain - and they would complain whether it was $70 or $700! My instinct is to say that once you start trying to explain to customers why it's so expensive, they'll just get defensive and argue with you. You don't need that stress, so just focus on being the best mechanic you can be!