There are a few things to consider.
1. For starters, noone pays a retail on those kind of speakers. If a customer shows up at a dealer with $188,000 in cash in his pocket (-20% off) no dealer will send him off. That is a very rare case though, as most potential customers getting $200k speaker would expect the dealer to accept their old pair of speakers as a partial trade. In my experience, people willing to spend this kind of money on hifi gear don't want to waste their time selling the speakers. A marginal price reduction ($170k) will not change that.
2. The market for such a big speakers is very limited. Unlike megabuck DACs for example, which can be sold to an audiophile with room of any size (provided he has the means to acquire it), this product can be sold to a very likited group of audiophiles, with only the biggest listening rooms. Those are few and far between.
3. The top of the range models depreciate the most. I will give you an example of Accuphase. Their 2 years old, entry level preamp in mint condition can be worth ~ 60% of retail. Top of the range model from the same manufacturer will most likely be worth less than 50%, probably around 45%. We are talking about same manufacturer and same product category. Because of the reasons I have discussed in #2 - speakers are even worse. Wilson Sasha 2 can be worth 60%, but Alexandria X2 may drop to 40%. There was a pair of Alexandria X2 mk 2 (upgraded from mk1) on AgoN, in perfect condition, listed by the dealer for $58k if I recall correctly. That was 1/3 of the original retail (which I believe is around $190.000 or thereabouts) and still no takers !
4. Rockports are in general tougher to sell and depreciate more than say Wilson on Magico.
5. With the introduction of the Lyra, Rockport has shown where the future development of their designs will go. From this perspective, Arrakis should be considered the outgoing model, that sooner or later will be replaced with a new design based on the aluminium tech. It is very much what happened with the Wilson Maxx 3, after the introduction of Alexia and XLF (with the new soft dome tweeter). The sales tanked instantly, as everyone was expecting the new Maxx model with a simialer soft dome tweeter to follow.
Which is why, IMO the price around $100k is more realistic. But I wouldn't really be surprised, if he only managed to get $80k.