As with many situations, your location could make a big difference. In areas with a relatively low cost of living I'd think anything over that $18.00 per hour would be reasonable, especially if they have good benefits. If you're in New York, California, or Tokyo, etc. I'd expect it to be a good bit more.
Your education level will likely also affect the pay rate.
First off, don't sell yourself short. You've worked for them already. You have experience that is directly applicable to them.
I suggest you poke around glassdoor.com to get some ideas.
I was once asked this very question at a conference by an intern, I told her "What do you plan to do with the money?"
What do you want your life to look like in 5yrs?
Write down all your debt and figure out how quickly you want to pay it all off. That might also help you figure out how much you want to ask for.
And just to throw an idea out there: why go permanent?
Back when I first started, i took the first permanent job I could take.
Most of my friends all took contract work.
They were occasionally un-employed but they got to see the industry through multiple views.
Their pay was more but I got benefits.
But, it has taken me many years to catch up to the number of contacts they were able to amass in their contract work.
Looking back, though, I would probably still take the first job again.