When you don’t know what to charge you can always research online their are countless websites that can help you and just might have what you’re looking for example…
http://www.forbes.com/forbes/welcome/?toURL=http://www.forbes.com/2006/11/06/bostonconsulting-marsh-mckinsey-ent-fin-cx_mc_1106pricing.html&refURL=https://www.google.com.pr/&referrer=https://www.google.com.pr/
Always keep in mind that you should research thoroughly before making anything concrete or absolute.