A while ago, my PC enchanted his +2 heavy shield with the effect of a phylactery of undead turning. He paid 1.5 times the cost of a phylactery to do this. (It was to avoid having to choose between two useful items for the head slot. Besides it looks cool.)
Now, if he were to further enchant the shield to increase the bonus to +3, would he still have to pay 1.5 times the usual cost? Or just the regular cost?