IF [ad_cost]<=3000 THEN 'small user'
ELSEIF [ad_cost]<=6000 THEN 'mid user'
ELSEIF [ad_cost]>6000 THEN 'big user'
ELSE 'Null user'
Let us know if this help.
After trying the above statement, the result is still the same. I think it's probably because the filter applied on user_nick is avg(ad_cost) but the function I typed in calculation field is just ad_cost. However, if I typed avg(ad_cost) in the calculation field, the field becomes measure instead of dimension. As it becomes measure, after dragging it to color marks, it won't show three independent lines which indicate monthly ad_cost changes in three user groups. On the other hand, I can't use ad_cost >6000 in user_nick filter either. It has to be aggregate function....
Could you please share some sample mock up in packaged workbook? It'll help us to dig down your issue.