That sounds like an incredible niche. I think the two best ways to set your rate would be:
- Decide what you want your internal hourly rate to be and then quote project fees to prospects based on how long you estimate the project will take. So, if after talking to the prospect you think it would take you 2 hours to write the functional design documents and you want your internal rate to be $250 an hour, tell the prospect this functional design document will cost $500 (I’m just making numbers up here)
- Decide what the value of the functional design document is as part of a larger business analyst project. For example, if the total fee for the bigger project is $10,000 and your part is 25% of the value of the big project charge at least $2,500. You may be able to charge 5–10% more since most analysts hate to deal with this part of the project. (Again, I’m just making up the costs)
I hope this makes sense. I agree you don’t want to charge based on your time. You really want to charge based on how much value you are providing and part of your value is that you are good at something most everyone else in your industry dislikes to do.