
I find this quite hard to establish myself, however recently I've read a great blog post from Vincent Laforet on the subject.
http://blog.vincentlaforet.com/2013/08/ ... r-c-o-d-b/
It's about the C.O.D.B.
C.O.D.B stands for "Cost of Doing Business." Basically it’s a number that represents what it costs you to operate your business for every day that you work.
Which should allow you to establish a minimum of what you should charge to survive.
He even offers a tool to calculate it for yourself
https://www.dropbox.com/s/ulw0n15vnw0rsfo/CODB.xls