If you want harder content, it should be because you enjoy harder content and that in itself should be all the 'reward' you need.

As I've said in another topic, through various games, I've seen people say they want harder modes and more challenging content introduced because they enjoy pushing themselves and overcoming a challenge. Then when it gets suggested it have the same reward, the response is inevitibly, "WTF? Why would I do it then? If I can just get the same thing for doing the easier stuff, why would I force myself to endure something so annoying/tedious/difficult?" :-/