Unreasonable Prices of Per-Win IP Boosts (Let's do some math)
Normally you get about 80 IP per win and about 60 IP per lose. You also have first win of the day. So, lets say that you get 80 IP per game in average.
Suppose you want to buy a 6,300 IP champion. That means you have to play around 75 games to buy it. Or you can buy RP and get that champion for 975 RP.
From http://leagueoflegends.wikia.com/wiki/Boost : You get extra 158-168 IP for every win in ranked. Let's add these 80 IP in average per game. That means you get about 250 IP per game (I know it is actually less since you are not going to win every game, but that's OK for this example). So, you have to play around 25 games to buy that 6,300 IP champion. 25 wins IP boost costs around 1,500 RP.
Conclusion: You save more money by getting a champion instantly. Not by playing the game.