Lori,
There is only one way to charge for something that is open-ended or infinitely variable, and that is to charge by some unit that is finite and calculable — i.e. units of time, such as: by the hour, the day, or the week. When you buy eggs at the grocery you pay by the dozen. When you buy gas, its by the gallon. Like the grocer or gas station operator, you also need to charge a specific amount for a specific unit. In your case, its for your creative/technical time and abilities.
And, you should make it very easliy calculable — meaning, don’t get fancy and start breaking it all down into machine time vs. your time, vs. writing, vs. online, vs. graphics etc. For example, I have only two types of billing time, 1) consulting time, and then 2) all the rest.
Hope this helps…
DRW