As with everything at the moment, you need to see this in comparison with the rest of the market. And the fact is, that ...
Oh ... actually I can't finish that thought as the stats don't support it! Was going to say that the Kenner cards are much more plentiful and they currently have an average of £115, so it's fair for Mecanno 20bk to be £150+
But as some of you have said already, this Meccano 20back is the most common version you can get:
Above are stats for the last 6 months sorted most common (in terms of recorded sales) to least seen. The market is not always rational!