I've never seen them on sale because most stores consider them as gift cards, rather than accessories, like the 360 points cards. You will most likely never see gift cards on sale, because they are worth nothing until they are activated my the cashier.
I hear you though, I would really want to find some cheaper, I just don't think it's gonna happen... Something what I do is use my BestBuy Rewards $5 off to get a $20 PSN card, hope that helps.
I typically use Edge credit at EB games for my points cards since most of my Edge credit is always from trading in games for less than I paid for them.