Wazabi
Don't believe what you read. You can only generalise on price per carat of any gemstone because it will depend on cut, clarity, carat weight and, for Zultanite (Diaspore) the strength of colour shift. For example, a 5ct stone with great colour shift will command a far higher price per carat than a 5ct stone with very little colour shift.
There are very few gemstones (leaving aside diamonds) that can command the amount per carat you've quoted. There ARE some but Zultanite isn't one of them. It's not rare. It is sold via a limited number of distributors though and this keeps the price up to buy (like diamonds) BUT it's resale is low, unlike diamonds.
Here's a 4.64ct Zultanite set into a ring that has wonderful colour shift and clarity. How much did it cost as a loose gemstone? £200.