Here's the formula I used =
Bonus (5) squared (25) times 4,000 (100,000) times 2 (200,000)
Basically I took the formula for the Dusty Rose Ioun stone which gives a +1 circumstance bonus and costs 4,000gp:
1 squared = 1
1 times 2,000 = 2,000
2,000 times 2 (for an item that doesn't take up...