Naturally its supply and demand I guess. There is plenty of demand and limited supply so there will be times when they break out above their general value. You'll offload 1000 in an hour if the price is right.. no problems with liquidity. As the silver price is trending downward I would argue less people are willing to pay the premiums sellers are asking but that can change pretty quickly too when supplies run short.
If the price did get down to $15AUD then there would still be as many 50s around as there are now . It would be just like we have seen in all the past years. We may think that the price is going to rise and see GOOD BUYING VALUE, but there would be even more people out there that lose faith and sell before it goes to ZERO. $15 $20 $30 $50 it really makes no difference at all in supply, there will always be the stuff you want to buy at the price you want to pay. You just have to shop around and learn to tap different markets at different times.
They can't go to zero.. the have a hard price floor @ $1.50 per oz.. Ill buy all day at $5.. also when prices go way up these coins tend to get offloaded and melted.. illegally so the supply side shrinks..
Any time before 2008 you could have bought for around $5. Even in 2009 you would likely had a chance. Assuming you could have got them near spot. Were you a buyer then? You could have been in the 10k club since then - for all I know you have more than sammysilver. All I know is I would like to be in the 10k club.
I think you'd have a hard time sourcing them in reasonable numbers at that price. I have the impression that round fifties are held in very strong hands.