Interesting... and what exactly do you base this assertion on? I mean we can all assume what we want and pretend we're discussing, or we can look at some facts. If we don't have enough facts, we can at least study the current market and in this case the reason behind the prices placed on items.
Let's go along with your example. What sort of goods can we expect to find selling on the market for ~1 mil? Some materia? Uncommon/melded/HQ gear? New, high demand items that have just hit the market? For all of these things, how does one determine the value to sell for in game? Are they really based on cost?
On the contrary almost everything selling for 1 mil in game is selling for that price as a means of supply/demand. Supply/demand isn't primarily influenced by the good's cost when the scale tips towards higher demand and lower supply. No matter how high the demand is, nobody is going to be willing to continue to pay 1 mil for something when 1 mil becomes 10x more difficult to make. The sellers in turn are going to have no trouble
drastically lowering prices until sales start flowing again and a new value is found in the market that satisfies supply/demand.
So, considering how the market works at such high values, prices dropping from values such as 1 mil to that of 100k is not impossible.
It is inevitable.