You might already know this, while studying the 101-in-1 Games, I noticed theApp Store shows the top "In App Purchases" ranking too. It sort of provides you a rough idea about:
(a) whether the game provides In App Purchase
(b) what sort of In App Purchase options the creator provides
(c) which product/price range is the most popular one
Also have a quick look at another one of my favourite game Tiny Tower and it also shows similar ranking as well, but only 3 been displayed instead of 5.
This also proves you can indeed make money with free games too! And imagine those are your profits if it's your game... Got to work harder mate!