The most affluent classic car collectors typically duke it out in auction rooms to get their hands on the rarest, most exclusive and desired models from past eras.Aston Martin, Bugatti, Bentley, Ferrari, Jaguar, Lamborghini, Porsche, Rolls-Royce: just some of the names that traditionally command most attention - and highest bids - from the wealthiest petrolheads looking to add cherished motors to their garages.But when it comes to the cars posting the biggest spike in value in recent years, one mainstream brand is putting its lavish counterparts to shame - Ford.High performance 'Fast Fords' from the seventies through to the nineties era have become serious collector's items of late - and none more so than the Sierra RS Cosworth.The souped-up eighties family car cost up to £19,950 in showrooms four decades ago. That's around £59,500 in today's money with inflation factored in. But wind the clock forward to present day and well-heeled enthusiasts are willing to splash six-figure sums on the best surviving examples.Later this month, six Sierra Cosworths will go under the hammer at an auction in Northamptonshire. Collectively, they are predicted to sell for an eye-watering £1million. Which begs the question: how has the market reached a situation where an eighties hot hatch is worth the same as - if not more than - a Jaguar E-Type? The Ford Sierra RS500 has gone from 1987 souped-up family car costing £20,000 to a six-figure collectable today...