I got only 10 000 a year (i.e. 820 a month) but am living in Brighton, which is very expensive, i.e. I pay 350 rent a month. So if I get a postdoc anywhere else I will be probably better off than I am now.
err- no-idea, I don't know if your stipend will be tax free in the US. Did you try to look this up on the websites of the US universities you are interested in?