SEO Charges are really complex issue to discuss. They are many factors you need to considers before you start any SEO project.
What should I charge? - Well, I believe it’s really depends how much traffic you can delivered to clients. Also, you need to consider what is the value of client’s one sale. Value of one sale can be $100 to $100000. Also, you need to consider which business type(small, medium or big) you are targeting. Can they afford or not?
But, I believe best way is to consider your cost and profit margin. When you see scope of any project, you can imaging and calculate your time and afford to deliver X number of traffic and then you can add your profit on top of it.