$10.97 on the first full month i ran Adsense on my forum. But that's on a forum that was already running before that without Adsense for a while. Numbers from other people won't be any indication for what you will make your first month. It all depends on how much pageviews you have, your CTR, and how well your niche is paying.
My first month I ran Adsense I made a whopping $2 or $3. After trying numerous ad placements, colors, and sizes I've managed to bring in many more times that amount now each month.
it depends on how do you advertise , however inital months payment is always low than later payments .growth is always exponential
Forums don't earn anything, the CTR is very low. Your better off with selling advertisements yourself.