The Turboprop World-Beater VISCOUNT
Almost at the very end of that page is this:-
By the time Vickers ceased production of the type in 1964, a total of 444 had been sold to 50 major airlines, with a value, including spares, of £177 million, export sales accounting for £147 million.
So £177m in sales just need to find out how much it cost to design/develop and that would be a starter for ten!