I think this may be a "how long is a piece of string" question...
I went in The Works today and they had balls of really nice wool for £1 - that silky kind of fluffy wool, do you know the stuff I mean? How many balls would I need for a scarf (they were not very big balls and it wouldn't be a very long scarf as I am rather bad at knitting)? There were some gorgeous colours to choose from and some were very wispy kind of silky wool - the sort that knits kind of fluffy, and some were like silky ribbon-y kind of stuff.