Need to differentiate between "Preparation Cost" (before you enter the dungeon) and the In-dungeon cost.
Agreed, though to be fair I've made a forum post asking the devs just to get rid of the discrepancy altogether. The rule of thumb appears to be that elite items are double the in-dungeon cost, regular items are equal to in-dungeon cost --Darvin 12:34, 21 August 2012 (CDT)
The Crystal Ball seems to increase its conversion value as it gets charged. When prepped or bought, it has 10 charges but only 1 conversion value; however, after receiving additional charges, the conversion value seems to change to 10 points per charge received (up to maximum 50 points). After discharge this also resets back to 1. I haven't tested the Fire Heart if it uses the same logic.