Something additional that people wanted to see was information about codestone drop rates. That's not going on the page due to the inaccuracy that any sample would have, so I'm posting it instead - I remember there were a few people here who were submitting so here you are.
Notes: For this, I excluded all submissions that weren't 15 prizes at once, to reduce selection bias. I didn't differentiate between red and normal codestones. Also, this doesn't take into account the actual time it takes to get the drops (since some domes seem to be less willing to give loot in general).
So, in terms of the three difficulties (easy, medium, hard), for each challenger codestone drop rates were within 2-3% of each other, which is within a margin of error. As such, it appears that for codestones, difficulty selected does not matter.
Similarly, challengers within the same arena (for example, the Koi Warrior or the Walein), also trended to within 2-3% of each other. As such, it appears that challenger selected (within an arena) doesn't matter.
So what does matter? Well, we saw consistently different codestone drop rates between the different arenas. The results of that are:
-Central Arena - 11%
-Dome of the Deep - 30%
-Frost Arena - 17%
-Neocola Centre - 18%
-Pango Palladium - 14%
-Rattling Cauldron - 24%
-Ugga Dome - 14%
-Cosmic Dome - 15%
This means that, as is common knowledge, the water dome is best for codestone drops. But that it doesn't matter what challenger, or what difficulty. Essentially, if you can defeat the Koi Warrior on easy, you will be getting the same amount of codestones as beating Chiazilla on hard. Challenger specific loot will vary, but for most this is junk and isn't a good payout.