As always, any of these guns are worth what someone will pay for them at the time and if you find someone with a particular (and often sentimental) attachment to them then the price can be a fair bit higher than the 'norm'.
I paid a fair bit more for a similar condition Mk 11 in 0.177, with a particularly nice stock. Supposedly only 1 of 10 specially commissioned by Gamages of London - though I have no proof of that. The seller was an unscrupulous old sod who was actually playing me and another member, who was equally keen to buy, off against each other - despite the fact that he had advertised the gun at a lower price and I was first to say on thread that I would take it.
Under any other circumstances, I'd have walked away when he started conducting an auction, but I was flush at the time and, knowing how rare these guns are, I just couldn't resist. I would imagine that I could just about get my money back for the gun today - if there was a keen buyer.