This is a common question on the forum. The stock answer tends to be something like;
It is impossible to answer as pricing depends very much on the demand for your skills and the value of the dollar/pound/euro and other economic factors in the area that you are working.
My opinion is that the only common denominator is the fact that you have to make enough money to live on. If a client is offering you a one-off job, the rate for that should be at a premium, as opposed to a client that has an ongoing stream of work that will keep you busy for some time.