It's not about the cost.
If the modern gaming industry is any example, it's about control via some form of DRM and finding ways to monetize beyond the initial purchase.
Companies are increasingly controlling access to games - even single player ones - by requiring online authentication before you can play.
Some games are being sold now with a key that you have to register to play online...and once that key is used, nobody else can play online with that copy of the game unless they purchase a new online key (think: Gamestop & used game sales).
Games are being sold with locked content on the disc, that you unlock via DLC purchase. It's already there, you just have to pay extra to use what's on the disc you bought at the store (not a new tactic, but still annoying).
That's just the way things are going at the moment.
Or to put it another way...