Its a new champion, no new champion is going to come out for less than 4800 IP, most will be 6300 so the hardcore players can get the champ perma unlocked immediately if they wish, or spend their money on RP. As a F2P game they have to make money, they make no money on you grinding out games you arent paying to play and then "purchasing" champions. Yes it is a lot of IP, and can be hard to grind for some people and those of you who really wanted her and waited are frustrated as well. I respect the frustrations felt by those players, however it is rare in a f2p game that you can find a model where you aren't paying for power, i.e. aren't paying cash and able to say start every game with your AD carry with an IE. Just an example of how other f2p business models work, making expensive IP champs gives them the chance to make money other than skins for those who want immediately, those who don't can wait for free weeks or grind out the IP.
Again respect your right to your feelings people, but please having seen the model and their pattern, you have to respect them as a business wanting to make money to make game/servers better and stay around to even provide us with the game. Enjoy to those who bought Leona!