The Arkenstone is actually priceless… a value cannot be placed on it.
The Arkenstone is completely unique, only one of them exists as far as we know, and its uses are limited. It simply exists… and that existence is used by the Dwarven kings to secure their rule. That’s what it’s good for, that one thing.
So if it were to have a value that value would be set by the Dwarves, who find the most use in the Arkenstone. But the Dwarves never trade or sell the Arkenstone, besides the Bilbo incident, so no real price can be given to it.
The Bilbo incident is the closest we can come to a price, when 1/14th of the treasure of Erebor is offered in exchange for the Arkenstone. 1/14th share would come out to trillions of US dollars, as Erebor was insanely wealthy.
So there you have it, the Arkenstone is truly priceless… BUT is worth trillions in a single incident.