I suppose how much you want/get for it will depend on how much of a hurry you're in to sell.
Mine's been up for sale for what feels live forever - although I think it's actually 5 months). If I'd have wanted to sell it quickly, then a £17k tag would have probably seen it go very quickly, but I thought it was worth much more.
I've had people (i.e. timewasters) phoning up saying that they don't believe mine is worth what I'm asking for it as they've seen another for £2k or £3k less (but in Imola/Interlagos with bi-colour leather). My simple reply is to tell them to buy the other one then, to which their reply is "Well yes, but they're not in Ruby Black with Champagne, innit."
Despite the fact that I only paid £25k for my 9,800 mile model, 2 years ago when it was 2 years old (that's over 45% depreciation in 2 years - invoice price with options was £48k), I'm not planning on accepting some derisory offer. I've had plenty of 'offers' from £16-£17.5k, but I think those offers not only undervalue mine, but all the others out there, and this price will rise in the new year - if you can afford to wait.
I'm 'now' not in a hurry to sell, and even with 36k on the clock I think it's a bargain at £19k. I've not got any problem driving it in the winter, and it didn't have any problem in 4 inches of snow last winter either.