From what I understand the problem comes from PSN cards being actual "Money" i.e. giftcards, vs live cards, that are "Points". I've only gotten them at a discount through ebay (winning a bid at slightly lower than standard price) and recently through a coupon at HEB (Texas grocery stores chain) where they had 5 off 50, 15 off 100 and 50 off 300 on gift cards