I work for a bodyshop jobber and actually thinner costs and waste, as Ronny stated is where you save money with PPS. We sell them to probably 95% of the legitimate shops that we service. When they first came out, 3M had a disc and would record the guys in the shop and the amount of thinner, time and product waste in their actual shop. A quart of thinner, extra product mixed and thrown out, time, waste disposal added up to more than the cost of the lid and liner. They have other benefits as some mentioned, being able to spray upside down, eliminating the use of mixing cups (less product waste), not having to strain (less product waste) being able to cap and save product for an amount of time , etc. The less you have to transfer product from mixing cup to strainer to gun cup, out of the gun cup, the less product you have to mix up and you have the potential to save on product usage also.
That being said, there is not a lot of markup made on supplies. $70.00 is probably the best price you will find.