Correction that was £80 over book price, which is $150 over. But it's a nice French coin and i couldn't resist. I'd buy another one at that price too if it looked as nice, or if i could even find another.
Too "wide" with more people collecting and a limited amount of collectible coins available?? The fact of the matter is that a coin's value will always be based on rarity, grade, and market demand. Key dates and high grade coins are NOT the norm. Their numbers are limited not just from a low mintage standpoint, but also how many were saved/hoarded against those that were "lost". A limited number of a given coin coupled with an increasing number of collectors due to a simple growth in population all are simple elements that effect supply and demand prices. Now there have been exceptions. Before 1962, it was believed that a "handful" of 1903-O MS Morgans were available as values for these coins hit $1,500 for a BU specimen in 1962. That is 1962 dollars remember! Just after the vault find of 1962, these dollars quickly fell to $15. Why...the "handful" of the known population quickly turned into bags and bags of this date. But the laws of supply and demand with key-date coins pretty much make sense. Sell a very limited and given supply of key dates/high MS grades to an always increasing population base and prices will go up faster than those coins that are more available. Unless there is a hoard of key dates stuck in some bank, I don't see keys and high grades going down...unless we are attacked by terrorists who strive to destroy our economy and way of life.
I guess a point I was trying to make [with too many words] is that sometimes quantity can equal or beat quality. I suppose you could collect every Roosevelt dime up to 1964 [including the 1964d double die] in MS65 for the cost of one 1616D Mercury dime in G-4 condition and have quite a bit of money left over to buy other things [like every other Roosevelt for example]. I don't think that it is clear that owing the one dime is better than the complete collection of another series. Quantity sometimes counts.