It's always a difficult thing to speculate on future values. The 20p mule had a mintage which put it in the same ballpark as 1950 or 1951 pennies, the early euphoria rapidly died down to produce a similarly priced coin - a not unreasonable outcome. Crucial to the observed pricing data was the availability of examples to the public, which immediately resulted in a speculative bubble before settling back once common sense gained the upper hand. The 20p error coins found their way into circulation, whereas I'm not sure the aquatics 50p did. All the examples known to this forum came from the early packaged sets, so there is no easy way for the public to gain exposure except for those sets that have been spent. This is not going to be a very large number for the forseeable future given the £60-70 loss an original owner would incur. Maybe future generations will spend them, assuming they are still circulating, but I don't think this would happen for a few years. That would support the price at the current level because there is no evidence to support a large population - people are too greedy to pass that one up. Therefore I'm not so sure that prices will drop so low as £300 if only a few hundred examples come to light. I have to confess that I have tended to look a little dismissively on decimal coins, but given decimal coins are approaching their 50th anniversary, then it is clear that many people know of nothing else. This was forcibly demonstrated at the Midland yesterday. Along with at least two other dealers in the immediate vicinity, I didn't sell a single discrete predecimal coin. What I did sell was hundreds of pounds worth of decimal sets, 50p's, £2s, modern silver proofs etc. Everybody wanted modern stuff, with 50p and £2 coins the most popular. I didn't see that coming and was fortunate to have bought a full run of sets only a week before, but had I not done so might have taken less than £20. The demand isn't restricted to UK 50p's either, as I am regularly asked for Channel Islands, IOM and Gibraltar 50p's too. The market for decimals is much healthier than many might imagine.