(Edited out exact amounts to keep within the restrictions of not discussing actual rates on this site -- NogDog)
If you're doing this as a career, I've found it's often best to work out how much you would be paid on a yearly basis, eg if you think you're worth xx.xxx a year (I'm working this out in GBP if that sounds a little low to some people), then that's approx £yyy a week which works out (if you work a 40 hr week) at £zz.zz an hour. Or $zz an hr.
Thats probably what I'd put myself out there as.
As for the per job vs hrly rate? Thats entirely up to you. It might be worth thinking about that on a job-to-job basis