From what I have seen, the TAship amounts and responsibilities can vary widely. Thus, in addition to considering the amount funded, you may want to ask amount number of hours work that will be required. I agree that somewhere around $15-$18K is probably accurate. However, some of the better programs have stipends of $22,00+. In addition, I know of students who have earned significantly more if they took a funded RA position and were a top recruit, but in general, these required many more hours than typical assistantships. Some fund students for the whole year while other only fund students for the academic year.
One other thing to consider is that some programs will guarantee funding until you graduate while others will guarantee funding for only the first year or two. Thus, it is important to read the program materials, as a previous poster suggested. Good luck!