Flat project fee?
Percentage of expenses?
Percentage of expenses plus hourly rate?
With regards to charge I went started looking at the going rates in my area, and how they charge. Next I reviewed the services they offered compared to my services (remember some established planners are not charging just for their service but their reputation as well). Next thank about the clients you are trying to services. After you've done your research really think about the cost :
Hired help etc.
I charge a flat rate, but its based on the number of hours and cost. For example If I charge 1400 for Paritial planning which breaks down to $70. an hour for 20 hours. Not 20 hours at one time but 10 hours for wedding day and 10 hours for the other time you spend on planning.
Hope this helps.