DB is correct when he suggests you may be able to do better with another dealer.
Generally when SP discontinues a model, they allow the dealer to sell that model outside the normal pricing restrictions. (I have gotten a lot of great deals that way over the years from SP dealers and a year or so ago bought a new Mk 20 D400 for 10% over dealer cost.)
The lack of a price restriction means the dealer could sell it at or even below cost if they wanted to. I know a few dealers who will stock up on discontinued items to allow them to have items in stock that they can then use to compete with internet retailers and discount brands.
I have no clue what the dealer cost would have been on Mk 18, but in general the scuba equipment industry operates on about a 100% margin, so figuring half of MSRP will get you in the ball park with most products from most companies. Unfortunately I also have no clue what the MSRP was on a MK 18 either.
And of course this all depends on whether another dealer ever had a Mk 18 or still has one in stock. If not, a dealer who still has one may be in, or see themselves in, a position to get as much as someone is still willing to pay for one. I've met a few dealers like that as well but I have never spent much money with them.
