I bought Dissidea 012:FF last night. I am thinking of getting PS+ today. There is a pretty heavy discount on Dissidea for PS+ members; would PSN retroactively put that discount difference in my wallet if I signed up today?
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chaos is 100% correct, there are no retroactive payments made in situations like this.
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As Chaos and Brit said, signing up for PS+ after you have already made purchases, will not provide you with any sort of retroactive discounts, refunds, etc. Don't let that stop to you signing up for PS+ ... give a 3 month subscription a shot ... as you saw with Dissidea 012:FF, the PS+ program often offers some very nice discounts.
PS - Welcome to the forum.