What Value To Put On Content?

When it comes to the content you use on free sites, galleries, AVS/AEN sites, paysite members areas, etc, how do you attach a 'value' to it in terms of what should/shouldnt be charged for a specific movie scene, image set or story?

Like is there some kind of formula to say X amount of pics / content cost = $X value that you use?

Without going in to to much detail, we're about to start charging surfers for specific image sets and movie scenes so we are not as dependant on recurring billing and 'membership' based pricing so im trying to get a hold on how to figure out pricing for this new billing method.

Regards,

Lee