[How to estimate price? By time? By picture? By character?]

Because I still doubt about pricing. Sometimes I have a project with 10 pictures and 2 minutes long. Sometimes 5 pictures and 6 minutes long.
Some customers have source files some of them not.
Maybe some of us adjusted proper formula for such orders. 
Thank you!

[How to estimate price? By time? By picture? By character?]

Hi
A customer wants me to animate 5 pictures(png format) with some text on them. The total length is 2 min.

What is the most fair way to estimate the total cost? Should I ignore the length and count by pictures only? 

There are no any layers so I need to split up all objects and characters and redraw if it necessary.
What would you charge for that type of project?
Thank you.