Ultimately whether a set good value or bad value in isolation is irrelevant, because it isn't sold in isolation. It's a digital decision, of value to the individual . It is either priced at a level combined with other factors such your interest in the theme / set etc, that you either buy it, or you don't.
For example, the Disney Castle I wouldn't buy even if it was at 100 quid, nevermind 300. Because of other factors such as the above.
So in terms of value, the Disney Castle is terrible. To me. No one can make a claim on value outside of its value to themselves.