There is a theory that real prices can be measured "across the ages" in Mars Bars. The Mars bar is made from cocoa, sugar and fat, and if "the market" works at all, those are commodities that it will work on best, so at any time its price represents the real value of money. It's been applied to such things as "an entry level car", "a gent's two-piece suit", "a graduate entry salary at ICI" and many such things, with impressive accuracy. A Mars bar in pre-decimal 1948 cost 3d (and came in three flavours). Assuming that it didn't rise in price by 1950, a Black Shadow would sell today for £375 x 80 = 30,000 Mars Bars. Guessing that a Mars Bar today costs 50p, that's £15,000. A Comet would sell for £9,600.
I still have a 1954 book 'British Motor Cycles of the year 1954', price 2/6; publisher Stone and Cox. Ltd. 44 Fleet Street, London E.C.4 (central 1622) The Vincent models are all series 'C'. Prices:- Comet £258: 0 : 0, Rapide £326 : 8 : 0, Black Shadow £366 : 0 : 0 and Black Lightning £474 : 0 : 0