Those things can be surprisingly expensive- don't specify them without finding out, first. You'd think a chunk of rubber the size of a tire would cost about the same as a tire, but not so.
They cost more than a tyre because they are hand laid up. They are an engineering product that works.
The price of an article is soon forgotten if it performs. When it fails then you wish you had invested in the real thing.
Compared to a flap valve that requires an nin ordinate amount of maintence they are cheap. Do your NOV on the item. OK if you are in the third world where labour is cheap but with all the OH&S issues hire wages etc the cost is worth it.
