Funny, I bought the game at Best Buy, and it did cost $40, not $50.
However, the game is supposed to play completely differently based on which mode you choose: It's actually a difficulty setting. Muso mode has you automatically block attacks when possible, and the amount of damage enemies do is relatively minimal. Shura mode is by far more difficult, and enemy groups can deal anywhere from 1/4 to 1/10 of your health per hit.
Either I'm terrible at the game, or when I enter a new area, enemies are actual threats to kill you. Since you instantly revive in the same room you died in, they're no real threat, but still, they can do great damage. Those monster lairs with 100 enemies really require you to identify and understand the best way to avoid one particular kind of enemy, which can be very difficult in some cases.
I'd say it's worth it, so long as you don't play wimpy on Muso mode, and instead choose Shura.