In Phantom Hourglass, the Regal Ring is a rare item, and can be found in stores or given to Link by Nyave. There is no set price for this treasure, and there is also no limit to how many there are in the game.
In Spirit Tracks, the Regal Ring is first introduced to Link when he meets Linebeck III, the greedy grandson of Linebeck from Phantom Hourglass. He gives Link a letter from his grandfather, explaining details on where to find this expensive treasure. When Link finds it, it allows him to pay the Bridge Worker for everything that Linebeck III owes him. After this, Link can find an unlimited supply of Regal Rings in the Lost at Sea Station every time he beats it, as well as prizes in various competitions. These treasures can be sold for 2,500 Rupees.