I have worked off and on for many years as a contractor working on high-end database scalability and performance issues as well as parallel algorithm design. While the rates fluctuated with the market and how much I liked the project, I was always charging well above $100/hr even for terms that ran several months.
A lot of it has to do with who your customers are. If you are working on the design of a system that will be moving $250M per year if it performs correctly, spending an extra $100k to hire a deep expert in the design of such systems is below the noise floor. You might be a "database scalability consultant" but you are not fishing in the same pool as most of the people that claim similar labels for themselves.