With 15+ years experience, I charge $75—$150/hr depending on the type of project and amount of effort required.
When I was doing freelance with only 2 years under my belt, I think I was charging around 20–30/hr.
I rarely do flat prices, but that’s due to the type of work I deal with. However, I’d determine a flat price based on the estimated number of hours it would take to complete, with a little extra padding for those extra revisions that might occur.
But first I would determine an hourly rate for yourself based on what you feel your skill is at. Also account for the scale of the business, if there is a concise strategy already developed for the project, the deadline date, and the potential of future projects.