In my experience, no, 0.50 is not exactly great earnings for a forum. Keep in mind that it all depends on the forums content. Secondly, you can not completely control what 'content' people will discuss, and you will get ads for a large variety of keywords. Due to this, the ads may not be very relevant to your users, and will result in less clicks. Develop a forum that is highly targeted to a niche market, and you may find yourself with better earnings.
And remember, a forum gets a LOT of impressions. So even a little increase could get you a lot of money.
Forums are generally considered to be low earners, but that seems extremely low. You don't get anything for impressions, only clicks. How many clicks did you get for those 1000 impressions?
currently, google stop counting those invail impressions, so repeative impression from the same visitors won't count.