Auto repair

Why is it that when you go in to do something small (oil change), it ends up being more than that? Problem is that sometimes there are other things that need to be fixed, but sometimes they use our naivety to earn a few more bucks. My $22 oil change just turned into $200, but I do feel better about my car. Did I get scammed? Maybe, though those belts they showed me were almost shredded. Anyway, I guess it's worth it for the peace of mind but would sure be nice to know someone who had auto mechanic skills!