I charge $250 an hour for interaction design and general consulting (e.g. pure advice), and my husband charges the same for his expertise in JavaScript.
But I'm booked solid so that must mean it's time to raise rates.
Lots of good people undercharge because they know how much they don't know. Consequently, lots of bad people overcharge, because they are overconfident (or simply shysters). The trick is in knowing which you are.
I follow 2 key pricing rules:
1. Tell the potential client your hourly rate, then slap them in the face. If they're not more shocked by the rate than the slap, you're not charging enough. (Figuratively. Haven't slapped a client yet. Although some have deserved it.)
2. Remember it's about value and not about cost, just like those tire pumps at gas stations charge you a buck or more for air -- the air compressor doesn't cost anything like that much to run and maintain, but boy is it economical when you consider the wear it saves you on your $150-a-pop tires. Does an hour of your time only generate $50 of value going forward? Well, there you go.
FWIW I am in the midst of writing a short ebook filled with advice on consulting aimed at designers & developers, that I've developed over the past 11 years of consulting. Watch this space, kinda thing. Or if you'd like to "beta test" it for me, email me at amy@slash7.com.