I am open to proposals and negotiable fees depending on the job. My basic page rates are based on 50 page manuscripts or more; with a 50-page minimum. White papers and synopsis and cover work is still a flat $50 even though they are short.
Usually I put in a placeholder bid until I am more familiar with the job and the client and see what he or she really needs. Some people are open to suggestions and some want things done a specific way, so much depends upon that, as I mentioned above. I work in Word with track changes that can come back to you either as a final manuscript or with red-line so you can opt to change any of the "changes" I made, which I will do and then send it one final time in black.
As for writing jobs, my bids usually go by the size and scope of the work involved.