Banner advertising pricing

There is in the banner advertising means such a method of payment, as a fixed payment or Flat Fee Advertising.

So, I came across what I want to understand - how does the pricing of such a method occur and decided to turn to Habra in order to find out from the pros at least general principles, and the maximum task is the formula by which pricing can be done.

Someone says count, determining the price for 1000 impressions, but then the question arises, how to understand the price of these 1000 impressions ?!
You can count on CTR, but there are already very large errors and again you will not really understand.
And can hold the ratio with the prices of others?
')
There are a lot of questions, in fact, as well as figures, it remains for someone with experience to explain what and how;)

I still have little experience in this direction, but I can get the most valuable experience only from people who know everything in practice, alas, the articles did not lead to anything traveling.

I will be grateful for any answer, and thanks to those who will direct the brain in the right and right direction :)

PS Someone will say that the information is confidential, know the problems, then at least tell me how to think, and then he himself;)