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.
My wife works at one of the largest law firms in the world, and trainees there makes about $10-$15/hour. On paper their salary is high, but the hourly rate is pushed far down because of ridiculously long hours. Their secretaries make as much or more than them.
Newly qualified lawyers there make $20/hour if they're lucky. This is in the UK, specifically in a London "magic circle" firm - one of the highest paying law firms in the UK. US firms in London pay up to about 1/3 more total yearly salary, commensurate with the same firms salary levels in the US, but also tends to expect even longer hours.
It's first quite a few years post qualification that their salaries get high enough that their hourly rates starts to become decent.
Average salary for a solicitor in the UK was around $37k/year a couple of years back, for longer than average hours, at a time when the national average salary in the uk was ca. $40k/year...
I'm sure there are lawyers who start out at high hourly rates from day one, but I'd be surprised if more than a tiny fraction starts out around the $100/hour rate.
Actual billed out rate from a major law firm, sure - my wife is billed out at around $300/hour as a trainee.
You can almost certainly make more on a per-hour basis as a contractor in the software sector, but lawyers are risk averse and prefer a stable flow of work and decent health insurance. :D