its just a agent you add to the lacquer (or solid colour) depending on how much you add it changes the gloss content of it! it will polish up once dry! so if you have your car sprayed with matt lacquer it will stay matt but the more you rub it the more it becomes shiny !! as for cost its just a bit more to the total cost of the respray!
Some of it is in the paint, some of it is in the clear to give the matte finish to the car. You can also wrap your car in matte film. It's less than a proper good quality paintjob and not permanent. If youre tired of it, you take it off. It helps protect paint from rock chips and other abrasions, plus if theres light damage to a panel, you just rewrap that one panel and no paint needed