For a full length technical whitepaper with diagrams, typesetting and review, I charge 2.5 BTC. I can also do a quicker 'Position Paper' if you want to test the waters for quality and originality.
You realize that 2.5 BTC is worth $17,500 at this moment, right? I've seen 3-page "whitepapers" for ICO projects that were terribly written and incredibly vague on the details, and they raised millions of dollars in funding. Any competent developer team should be writing their own whitepaper anyway. A whitepaper should be highly technical and should provide some proof of concept. There's lots of room for marketing in an ICO project; the whitepaper is the wrong place for it. If the developers aren't capable of producing a technically compelling whitepaper, they shouldn't be trusted to develop a project.
And if a team is competent, they are much better off putting 2.5 BTC towards bounty campaigns to spread awareness of the project and aid in initial distribution.
Seriously, though, 2.5 BTC?! $17,500?!
I have two (quite successful) whitepapers under my belt. I can write your next whitepaper.
I imagine that anyone paying you tens of thousands of dollars to write a whitepaper is going to want to see samples and proof that you actually wrote them (like confirmation from project devs). If not, how is anyone supposed to believe in the quality of your work?
I can't put out that many whitepapers in a year. My process is slow and deliberate, and I deeply care about my work. Academic research is not a commodity mate, and if priced as such, surely is not research. 
