Cabrio in northern Europe in November ? Value ... er.. don't hold your breath! Mid-summer in Monaco, substantially higher.
Value on anything unique is highly variable and dependant on many factors, not least time, place, who happens to be in the market on the day and what other options might be open to them at the same time and place (and that's something you have no knowledge of or control over). Only real way to find out is at auction.
Why not put it on eBay at an undisclosed ridiculously high reserve price and see what anyone is prepared to pay. It will cost you the fees, of course, but any reliable valuation will always cost you a valuer's fee in one form or another. And if someone is prepared to offer a price you're prepared to accept, then you can sell.