Tannoy Monitor Gold, Red series value$appreciation

Many people say that the Golds and Reds are must haves for any Tannoy fans. Not only because they sound good, but also due to limited quantity available on the market, their values would go up day by day. Past data on price trend is hard to come by, I would like to ask how fast is the value goes up,preferably with a figure saying an approx. rate of increase in annual term.

Thank you.
Chris