Which country and sector?

Your experience, employer's turnover?

That's like asking how long is string. There are way too many variables like location and the going rate for professionals with similar experience.

Since you know what your salary currently is, start from that. Then factor in the loss of benefits (medical and dental insurance, paid time off/sick days, other fringe benefits), the shifting of taxes and insurance (for example, in the U.S., you would be paying all of your payroll taxes instead of the company paying half, employers don't have to pay unemployment insurance for contractors, etc.), and the fact that you can now more easily deduct more related expenses.

>>I'm starting a new position, but I agreed to consult for my current employer

i'm pretty sure your current employer would expect to pay pretty much what he pays you now (on an hourly basis) when you work as a consultant for him/her

All right ...

I'm in Central Texas. Currently I'm grossly underpaid, which is the reason I'm leaving (less than 20 bucks an hour - laugh if you must), so I can't use that as a basis. My current employer tried to keep me there (offering to match the pay and such). It was pretty embarrassing actually. Too little too late I'm afraid.

I'm the only guy there doing what I do, and they will have a hard time replacing someone of my caliber according to my boss. Anyway, I was thinking in the neighborhood of $65/hour. Does that sound outrageous or reasonable?

that sounds more than reasonable, and before you settle on that remind yourself that you will be sacrificing nights and weekends for this guy.