Yeah, yeah - League of Legends is a free game. Let's be real here, folks: you've spent money on it. So let's just fess up to it: how much money have you spent on League of Legends so far? Skins, champs, boosts, whatever you can think of. Now, of course, you can't give an EXACT amount since that isn't tracked or anything. However, just take a look at your account and give an approximate amount if possible.
Personally, I know I've bought a lot of skins. A LOT of skins. I own at least 1 skin for every champ that I own (and I own all but 10 of the champs right now). That's anywhere from 520RP - 975RP a piece (around $3 - $6 each). I also own every skin for Annie (except Goth), Malzahar, Fizz, Volibear, Cassiopeia, and a couple of others.
Therefore, given that I've only ever bought skins, I would say I've spent roughly $600 - $700 on the game so far.