You can surely make $1000 per month from one forum. But choose the topic wisely. If you go with webmaster forums or anything related to it. Then the CTR is very low. Because most of the webmasters don't click on ads. So try some other group of people who are available in plenty and can be targetted easily.
Depends clearly on the niche that you have selected for the forum & also your monetizing methods. Trust me adsense is not the only method to make money on the net.
I'd say that yes, it is, but it wont be from adsense! There are some interesting non-traditional paid forum concepts out there, and i'd recommend going that direction. 100 people paying $10 / month is $1,000, and i've seen forums that change this amount with over 12,000 members....do the math
read the G heat maps published, use your full creativity go go beyond basic template look and make your templates look like unique content pages to use normal heatmap display the 3 G-published successful formats use ATF on each page then you should get a ctr around 10% or higher even on forums - depending on topic of forum hence with 10'k unique visitors/day you should be in the 4 digits $/m my own recent experiments with ATF have clearly shown that the 336x280ATF pays up to 100% more than the 300x250ATF despite the fact that G loves us to place the medium rectangle ATF see also http://forums.digitalpoint.com/showthread.php?t=231312