From a mechanical standpoint, I think systems where magic has an in-combat risk have balance issues, especially if randomness is involved. Because a magic price could take the wizard out of the fight, you have to make her spells correspondingly more powerful.
If I made a "magic has a price" system, I would delay any consequences to after the combat is resolved. Let combat play out with mage spending her spells as necessary. Then after the combat she pays the price.
Of course, this changes the effectiveness of prices. HP damage is less effective, because it only has an effect if it does enough damage to kill the character. So you'd probably have to use some other penalty.
If I made a "magic has a price" system, I would delay any consequences to after the combat is resolved. Let combat play out with mage spending her spells as necessary. Then after the combat she pays the price.
Of course, this changes the effectiveness of prices. HP damage is less effective, because it only has an effect if it does enough damage to kill the character. So you'd probably have to use some other penalty.