I want to sell my forum..

100 of keywords get the site on google first page

most of company names in australia + complaint keyword gives me lots of traffic

76% from google and $150 each month from adsense

Depends how long it's been earning $150 a month.

Probably somewhere around $2-3k though.

Also - when you come to actually selling it I would also look into approaching some competitors (there's loads of complaints boards out there - I did some research on that a few months back) as odds are that many of them know a lot better how to monetise this kind of traffic and would therefore be paying you based on your traffic, rather than the current earnings, possibly making you a whole lot more than you would by listing it for sale publicly.

Just an idea