How much do I charge a client for a marketing plan?
Refer that service to someone who does it for a living full time. Since you mentioned a few key areas of service, whatever the resulting plan covers should also cover what you approached them to do in the first place.
That said, pricing on marketing plans vary widely depending on many factors-- the size of the client, their goals, their geographic footprint, their annual sales, their marketing budget, their degree of commitment to business growth, their tolerance for risk, their prior initiatives, their market survey requirements, how much time the planner will need to invest to completely cover all these elements, and the planner's degree of expertise in this field.
Without that info in hand, it's impossible to make an appropriate recommendation on plan pricing. Again, I'd recommend sticking to your specific area of expertise and reaching out to a marketing specialist to produce the actual plan.
Best of luck with this prospect!