What sites do you know of that always have discounts? I know of gog.com and g2play.net but I'm sure there are more. G2play.net especially usually has huge deals.
Have you tried Gamestop or Amazon? I've seen them offering old games for quite low prices. They sometimes have timed discounts on new games too but only for a short while so it takes luck and repeated checking. Amazon is overall better for deals on games, consoles, and etc.
I find that Amazon tends to have the best prices for shopping online for games. However, I can always find them a lot cheaper by visiting the local pawn shops.
Amazon is good, but for example G2play that I mentioned has deals of upwards of 70% off every day, Amazon has one of those deals maybe once a week or a couple times a month.