The kind of niche determines the cpc and if it is on an average 10 cents per click, you will be getting $40. For high paying niches, the return would be many times...
5orst case= 0.03 cents (avg) * 400 ==12 $ medium case = 0.90 * 400 == 360 $ good case = 2.00 * 400 == 800 hope u get the good case. it is prob related to traffic too.
This depends on niche and ads buddy . If it is a general niche than you can earn around $35-$50 and if it is related to finance , market , real estate or health than you can earn upto $80-$150 may be more than this too .
Niche is not all - the eCPM for each page can vary greatly. Have channels for individual pages if possible (if not too many). I discovered that the eCPM on one page could be 15 times higher than on another page. So I simply made an "internal ad" on the homepage and linked it to the page with the highest eCPM. That made a nice difference! Some good internal linking in text areas and a modification of the menus made the impressions (and earnings) raise even further on the best eCPM page. Try it yourself - it might be worth your while...