This is a very interesting scenario. It sounds like you did a lot of back end work and did not get paid for it. You do not mention exactly what your credentials are, or exactly what kind of service you are delivering (sales support, business development, media buying, art direction, graphic design?), but if you've never done a marketing plan I'd hesitate to encourage you to attempt it.

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!
Pattie Simone