One shortcoming is that they usually use the same size CPVC as they would if they used copper. But since CPVC has a thicker wall and the outside size is the same a copper, the opening is almost one size smaller. Because of geometry, the opening’s area reduces dramatically, this makes the pipes capacity almost half of that of copper. The cost is whatever he wants to charge, since he is the only one that will be permitted to do the work until you own the residence and by that time it will be too late, and too expensive, to convert to copper. We encounter this in commercial buildings quite frequently. The installing contractor can over charge up to almost what it will cost the tenant to install it himself afterwards and it will still be cheaper for the tenant to pay it.