Not long after I got my VIP and was able to mine them I sold 8 of them to someone willing to pay 600 each, I had no clue of the prices on a set scale; I guess we both had no clue who was getting ripped off at the time.
I agree with the demand statements, you can have a set price but if the demand is high, people will find a way to pay more to assure they're the ones getting it instead of a regular paying person.
Generally if there wasn't a map corruption issue / mines were reset a lot more than they are, you would see the prices drop a lot.
Keep in mind when the new map is out, diamonds will be a lot more available based on the fact it's new land, hopefully corruption issues will be minimal, and mines can be set up better and regenerated more than they are now on a weekly basis hopefully. Also, there are more & more donators every week, so that means even more diamond can be collected from the wild and non-permit mines.
People will still try and use this pricing range that everyone is used to though, for the players not thinking about these things, they'll still be willing to pay that price. You won't have to, they'll be cheaper once the map gets rolling - trust me.
(The sooner you can start investing in collecting them, the better - due to economic inflation as time passes)