It's expensive because its quality. I've heard of other manufacturer's, I want to say Taurus, cheaping out more and more on their revolver lines and have quality control issues like the finish chipping off quickly. That, and its got a ~$300 laser grip on it. Your J-Frame 642 is an older design, probably doesn't have a trijicon sight, probably made out of a cheaper, heavier metal, etc. Also, .357 has to be made stronger for the higher pressure. Basically, anything M&P is going to be made better which means more expensive.
The pocket holster has a nub on it that is suppose to catch on the edge of your pocket when you go to draw it. It takes some practice to get right, as you kind of have to slide the whole deal along the side of your pocket to get it to pull out right, but it works and can quickly become second nature.
PS: I still have it for sale, feel free to make an offer. I may even work out a package deal for someone if they also are interested in my Beretta 12 gauge I have listed on here.