Depends, If you can beat a game less than 10 days, you are saving money on even the cheapest games available. Considering Redbox is usually updated with the latest releases, you are saving money on virtually almost game you rent vs. buying. That is unless your someone who is my cousin and is OCD about having the actual game sitting on his shelf for the rest of his life.
Otherwise, at a game rental store, it costs about $6-$7 to rent for 5 days. I don't know about you guys, but many games don't take the full 5 days.
I do think that $2 is expensive, it should be about $1.50 a day. But on Summer vacation, I'd probably beat games faster than 3 days if it isn't anything stupidly long and I don't really go for trophies with PS3 games.