£35 for a plastic bottle. 
I believe the yellow cap comes at extra cost.

About another tenner, but they're shiny and new 
So £45 for something that probably costs pennies to manufacture. Crooks.
Almost as bad as the cam cover gaskets.......£50 for a few strips of piss thin rubber. 
You could look at it that way, or you could take the view that the water bottle with new sensor will keep the engine from over heating and seizing should the water leak out by telling you the level is low before damage occurs. (Although with the leaky cooling system on omegas, there's an element of crying Wolf about it as well)
Same with cam covers. They should keep the oil out of the plug wells and keep your ignition system from being destroyed.
Both are investments in reliability imo. And in the grand scheme of things, not that expensive.
These things don't last forever Dr. High miles and a hotter than most engine bay mean wear and tear.
One must spend some of ones millions to keep one in the manner to which one is accustomed. Mustn't one. 
Agreed

. Whilst it may seem expensive for "a bit of moulded plastic" (the coolant bottle is quite a complex moulding, with 4 internal compartments, and then the sensor and its mechanism), I couldn't fabricate (or get fabricated) for the cost of the GM one. Likewise with the cam cover gaskets, I could buy the cheapies (£15ish), but its 2-3 hours of my time every time I have to replace them (and many of the cheapies won't last the year, where as the overpriced GM ones (about £25+VAT) do seem to last several years.