I have seen rates in the interval from $60 to $500 per hour. The higher rates are more typical for high-end consultants who advise you on what to do (and you then get lower-paid staff to actually *do* it).
The Human Factors society publishes a salary survey in their Bulletin every year. They survey a sample of their members so it won't be HI Designers exclusively, but may be close to what you are looking for. There are many variables that make it difficult to boil down to one number. If I remember correctly, they report salary by degree, number of years of experience, and industry.
There was an articile in the Human Factors Society Bullitin about 3-4 years ago which gave specific data.
In general, and damn roughly...
<3 yr with an MS - 35-45k
5-10 with an MS - 45-60k
10 with PhD - 65-75k
Heavy weight 80-120k
More by 10-25% if in urban area (some will throw in a handgun, or at least mace).