Value is relative. Suppose you made about £5,000,000 last year---would a Martin 5K be worth £5,000 to you? For comparison, many people in the States who make about $75,000 per year (a solid middle class salary) will gladly pay $500 for a Martin 0. Each used ukulele is worth exactly what someone is willing to pay for it, but it's difficult to imagine that a person making $75,000 per year would spend $8,200 or so (equivalent to £5,000) on a ukulele---that's more than a tenth of her or his annual earnings. For the vast majority of ukulele players, all Martin ukes are too expensive to consider, anyway, and the question is academic.
I'm lucky to be a middle class guy, and I absolutely love old Martin ukes. But it still took me a few years to convince myself that a vintage Martin 0 is worth $500 to me. I'm glad that I did---I bought one a couple months ago and I'm looking forward to years of enjoyment.