I guess whilst quite a bargain they are at about the price they should be as they were always at a premium due to the branding. As time has moved on there are now alternative cinema style high performers at much better prices.
MK speakers are still decent speakers especially the higher end ones, I still have some myself (MK 651's) and owned a few other models over the years, I have never bought them new though due mainly to their prices.
I think their prices seemed to ramp up around the time of the 300 series and X subs.
The dealer in question as far as I know frequents some other UK forums, but not AVF, but I might be wrong. I bought a Hegel H360 at a great price some years ago. I’m not sure they still do Hegel. If you see inside the store don’t be put off if it seems a bit full of gear. Those prices look fab but no MP150/300s unfortunately.