Freelancers: pricing structure options

My role as a freelancer means I work on a wide variety of projects. From one-off infographics, to long-running research projects. I've found that there is not a single perfect pricing system - instead I make a decision on a project-by-project basis. I will ask myself:  - is there a defined outcome / product? - have I done anything like this before? - how easily can I estimate the hours I will spend on this? - is there a risk of 'project creep...