I already have a regular sealed version of Dead Island for the PS3 if I never plan on player the multi-player is worth even upgrading to the GOTY edition?
Well the GOTY actually just adds a few things you can use on single player also. The Ryder White campaign and the bloodbath arena which can be a good place to level up. I just bought it again at $20 so I am returning that copy and getting it for $8. You can still get $8 from GS I think (only $6 from BBY) for Dead Island so it would not really cost you much to upgrade if you cannot return your copy.