That is a phenomenal price hike, and Bailey should be ashamed of even suggesting such a monumental charge for such a basic component.
Unfortunately Surfer, I know from experience that many parts are specific to the caravan market, rather than being the common-all-garden domestic parts. they may look similar but frequently they are not manufactured to the same domestic standards.
That has been one of my biggest gripes with caravan manufacturers that rather than use the well tested and made by the million parts from general suppliers, they have very often decided to get their own equivalent part made, and as a result they run into supply and reliability problems. The uniqueness of the parts means they are more expensive than their domestic equivalent, which has a knock on effect to the quality of the product. Often not as well made to keep the costs down.
Taps are a classic example. when you consider how often a domestic tap is used and how long they last, why do we have to put up with taps that leak after 12 months, where you cant simply replace the cartridge with a domestic one because its a 'special caravan' sized component, and often made by companies that have no connection with the main line domestic suppliers.
Caravan manufactures may respond by saying they do this to save weight, and such moves may save some weight, but I guess that if all the special caravan parts were replaced with normal domestic parts I doubt it would make more than about 10Kg or less difference to the ex-works weight of the caravan.