yes the main cover is. its about 4ft square. the inspection cover however is about ten inches square and uses a simple sheet gasket which is easy to make.

because the main cover gasket is moulded cork or felt hence the price. also rarer it is to replace the part the more expensive. rad caps for d46/7u are au$150.
when dad bought the one for ours i swear i heard mum yelling about the price and i live a few blocks away from them. yet common arts like the button type grease fittings are cheap at under 10 bucks each.