My opinion is that too many people ordered new 720s believing that they would lose very little money or even make money they were caught up in the febrile atmosphere surrounding certain exclusive Porsche models which led to Porsche producing ~5000 GT3RS.1 models a huge number for such a specialist model, even in May this year when I bought my 720 it was conditional upon me signing an agreement not to sell it for a premium in the next 6 months this is quite illustrative of the atmosphere in which they were flogging them.
In the old days even as recently as 2009 Porsche sold 62 997GT2s to the UK market,(1260 worldwide) anyone who wanted one could buy one in reality but of course because the cars depreciated normally the real demand for such a car was low.
So lots of people queued up to buy the 720 and were graciously given a slot and quite simply I think a high proportion of these people couldn't really afford a normal depreciation £250K car and as soon as they realised there was no premium they bailed leading to a massive over supply of used cars. In the UK there are 55 720s for sale out of production of about 300 units.
The lowest price 720 in the UK is a Luxury launch model at £186K and there are plenty at around £200K with low miles I would be surprised to see them dropping much below £150K for the following reasons. Production of the coupe is coming to an end so in the UK we have around 300 units which is about twice the number of 991 GT2RS so quite limited.
The 570 model must prop up the 720 so how low can they go and what differential will be maintained.
The third element will depend on how well Mclaren network maintains and looks after their cars and customers. The 720 is such an incredible car there will be decent sized pool of car enthusiasts who will want to own one at some point to experience the phenomenon.
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997 GT2 2014 3.9 Mezger, 800PS @ 1.2 bar
2018 McLaren 720S
993 Turbo, 2006 built 3.8, 577PS/797NM, 1440kg DIN sold to a worthy enthusiast.