I dunno how you work that out??? Its part of the subscription which you can get for about £30 for the year. I can find the game for £25 so unless you get PS+ just for this game, yes its cheaper, but seeing as there are loads of other games then that statement is nowhere near applicable. I for one have had great enjoyment out of my PS+ subscription with some great and surprising titles that I would never have played. For instance Tomb Raider, Limbo, Contrast, Farcry 3 to name a few are fantastic AAA titles that have been available.
It would be unwise to sell your PS4 unless you really have no interest in it. There are loads of new titles coming in the next few months such as Watchdogs, Murdered: Soul Survivor, Evil Within, Driveclub, Sniper Elite 3, Alien Isolation, Batman: Arkham Knight to name a few and they are only the games I am interested in.