Depends, you can get a few codestones per day or none...
There are a few arenas that give better prizes, the Pango Palladium for example could give you a Mystery Island Paint Brush or a Stealthy Paint Brush... and I know there is one that gives you Nerkmids and Armored Negg ^^
So... 25k-?m
And the only difference between difficulties is the NP Prize.