I am not an expert at selling characters by any means, but I will post a few lines just to get the ball rolling.
As far as a "X isk per million skill point" I don't believe you can apply any such logic to the character reselling market. Basically supply and demand governs this market as every other. I have however noticed that specialized characters will tend to fetch a better price than more general type skill sets. Examples are POS gunners, CovOps cyno capable characters etc.
My best tip when selling a character is to start browsing the character bazar and try to familiarize yourself with the current price levels. These may vary over time as well. When the time finally comes to sell your character have a look at similar characters and try to match their level somewhat... again, it's a complicated subject.