To answer your question, I would say you are being unreasonable.
The vintage market is nowhere near as big as it used to be and in general prices have come down a lot compared with ten years ago. But back then there was a chance you could walk into your local hobby shop and find vintage parts to resell or go towards your rebuild, and there was plenty of loft clearance cars being put on ebay. People used to find parts in hobby shops and ebay them which meant there was always plenty of parts available, now the big sellers do tours of the country buying up old stock, and old shops are shutting with the spares they have left over being thrown away. It doesn't help that ebay are trying to put people off selling on there by charging a lot more than they used to.
Prices reflect supply and demand, for example vintage-kyosho sells Kyoshos Equalizer shocks at £20 a pair, while he has managed to sell out of
gold shocks at £50 a pair. There was obviously enough buyers out there prepared to pay £50 a pair for Kyosho golds even though most of us wouldn't. Another thing to remember, for vintage-kyosho/vintageavante/tamkyo-reprodecals and Europro975/Tonys Tamiya Parts selling parts on ebay is a full time job and their main source of income, if they overprice parts then they don't earn a living. If something you want is expensive from the big sellers it is because there is a demand for them and the sellers don't have many left, and we are talking about a limited supply of parts left in the world.
If you want a part and the big sellers are expensive, then do what everyone else does and wait for the parts to turn up at a reasonable price, or think laterally. For example I wanted a set of gold shocks for an Optima, so bought a Mid with a battered chassis for half the price of second hand shocks on their own. At least with Tamiya and Kyosho there is a good chance the parts will appear, my Predator rebuild took me 5 years to be counted as finished and I'm still waiting for some more original parts to appear.