Although it's mostly true that newer champions are a lot more expensive, and many easy champions are cheap, there are exceptions.
Take Twisted Fate for your consideration. According to the game, he's one of the most difficult champions to play but goes for 1350 IP.
Even though there are some guidelines to what the price goes for, it's still like any store out there. They set the prices to what them seems fit:
- People want to try new heroes, therefore they go for 6300 IP
- Some champions might not sell much at all, therefore they might get reduced in price
- Some champions need to be cheap for players to start owning and mastering champions
- Lot of other aspects goes in as well. They don't sell new heroes for 100 000 IP for the same reason a grocery doesn't sell milk for 1 000 $
Basicly it's a bit like asking "Why is that product cheaper than that product?"