The description of the Coin of Power does not list any DCs for its powers. Therefore you need to use the rule on p. 176 of the DMG for determining Saving Throws Against Magical Item Powers. However, none of the coins powers match exact spells.
I would equate the Black Bolt to a Finger of Death spell (since that is one of the spells required to make the coin) since it is 7th level, Min ability score 17 it would have a DC of 20.
All other saves I would base on 3rd level spells because the caster level of the coin is level 5 and typically a 5th level caster can cast level 3 spells. This would make the DC 14.
