My largest "expense" is tax. As an analyst, apart from some travel and keeping a home office running (stationary, IT equipment, internet access, coffee jar) my gross margin in my business is very high – I have very few overheads. In fact looking at my accounts for the last few years it seems to be 10-12% excluding any salary I might draw (as opposed to dividends).
I then pay the corporate rate on the net operating profit at 21%.
My personal take from every £1 I charge then is about 70p – and then I pay whatever personal tax and NI is appropriate on that based on allowances and how much is as dividend or salary.
And remember, being niche doesnt necessarily mean being the best (although it helps), just being rare. In the kingdom of the blind, the one eyed man is king. Or think of Jones bikes 😉
EDIT: Oh and I would like to differentiate myself from the Management Consultants. I never produce wordy documents, only financial models, and they are very often tested while Im still on the job. If I'm wrong, I'm, going to be there to cop it in the neck! 😉