Even if a plate did go for £25k or so (presumably the motor isn't worth much) I wouldn't place too much emphasis on it, because in any market there will always be 'outliers' that don't really represent the true state of the market, whether a price is well below the norm, or above.
For example, in the former case someone might need a very quick sale, and clearly this time of year isn't a good period for selling anything, particularly since sentiment is always rock bottom in the trade during this period.
As regards someone paying well over the odds, this could just be because someone doesn't know the market for plates, or because the price of the plate is normally conflated with the vehicle and they don't know the market for the vehicle, thus it might look like they're paying over the odds for the plate whereas they're quite possibly paying over the odds for the vehicle.
Of course, people in the market - or commenting on it

- will nevertheless use these outliers for their own purposes, for example someone selling will state the highest value around, while someone buying will obviously exploit the lowest know price in their negotiations.
Which of course is all to do with lack of transparency in markets, and one problem with plate prices is that there are relatively few transactions, and the pricing information isn't readily available to buyers and sellers. Compare the market for stocks and shares, say, where brokers have the very latest information on prices on their computer screens.
But the 'insider information' problem as regards stocks and shares is also relevant to plate transactions, although it's probably better termed 'asymmetric information', ie where the parties to the transaction don't have the same information about the market. This is especially so where a few big players - the two in Edinburgh and the guy in Glasgow - dominate the market, and thus they're the ones who'll know the real state of play, but clearly they're not going to be entirely open about what's going on to people buying and selling.
Of course, there's also a wider asymmetry in the plate market in that many people don't really know the score, for example that the council can pull the plug at any time. Or that in Scotland the legality of it all is a bit ropey. Buyers aren't told that - they're told it's goodwill on a business, and that it's more or less set in stone etc.
For example, the chap from last weekend will have no doubt been told as such when enquiring about buying a Glasgow plate, but I'll bet his perspective on the whole idea has changed since reading the News of the World article.
But of course it suits the whole vested interest group to cover up the facts (aided and abetted by the councillors and politicians) but the bottom line is that to that extent the market is far from transparent and the information available to buyers and sellers is often asymmetric.
On the other hand, in some ways it's not the normal problem of asymmetric information - for example, the seller of a car keeping quiet about the fact he knows the clutch is on the way out, or that a house vendor doesn't let on that they've got the neighbours from hell - because in the plate market some of those spreading disinformation think they're acting in good faith because they genuinely don't know it's bent, or that plate premiums don't really represent goodwill.