Got my money's worth is 1$ per hour*
*Subject to replayability.
for me its a 12~ long single player that justifies a purchase. i would rather much spend 50 or less on that. 15 hours is easily worth 60 for me.
if its a 5 hour game with a pretty good multiplayer (killzone 3 for example) then its well worth my money.
Destiny is a 10~ hour single player. 5 strikes which offer massive replay ability and a very good multiplayer mode. over all i easily put around 50 hours into the game. so well well worth the money, even though i would rate it a 8/10.