This is a trend that's been going on for quite a while. When a new champ comes out, they will cost 7800 IP for one week and then drop to 6300 after that. I can't remember the exact reason why Riot started doing this, but I think it had something to do with trying to discourage people from buying the new champ with IP (unless they really, really want it) so that games for the first week or so upon release won't be flooded with people playing the new champ.
Also, whenever Riot releases a new champ, they SHOULD (don't know if they still do this) decrease the price of I think 3 older champions, which are chosen based on order of release. Doing this will make sure there isn't an overwhelming of 6300 IP champions.
Lastly, yes. you need 16 champs in order to be able to play ranked, and this isn't counting the 10 weekly free-to-play champs since you're not allowed to play those champs unless you actually own them.