What factors determine the base IP cost of a champion?

When League of Legends was released the champions who were considered to be the easiest to play had also the lowest IP price tag.

The reason behind this was/is to gradually get newer players to get more comfortable playing the game. For example, Ashe (450 IP) being quite easy to get the grasp on and Anivia (6300 IP) where more significant experience on the game is "required".

However as Riot progressed to release new heroes, they tend to be more expensive.

As of now, every new hero is priced at 6300 IP

The cost is just a definition of Riot. But generally older champs are/get cheaper than newer ones. New champions are 6300 IP as far as I seen.

My interpretation is that there are some cheap(er) champions so that newer players can quickly buy a new champion so they get something new to play with. And the more expensive champions are somewhat a "longtime goal". So the pricing is as is so players stay interested in the game.

The cost has 0 relation to the strength of a hero. Nearly every champion is on the same level, balance wise. Its just the skill-set/usability of a champion that get somewhat defined in strength; this can be seen in tier lists for competitive play. (But generally every champion is viable, period)