These are really far too rare to offer a guide price on. If you want one, you're unlikely to find more than one example for sale at any one time - which means you'll have to either negotiate hard or pay what the seller wants. You don't really offer much information as regards to year, price, mileage, condition, whether it's been restored etc, but there is a Zagato-style 12/4 Sports Special that's been for sale most of this year - VL6625. It carried an estimate of £55-65k at Bonhams Goodwood Members' Meeting in March 2018 and has been for sale for sale with a dealer at about £65k since.