Sweetboxing.com - A fast growing boxing forum, which is around 18 months old. - Current earnings : Around $100 a month with adsense, 20 Cents a day with a popunder company. And i think we did $200 in donations in past month. ( I divide some of that to admins and mods there). We have a very loyal community, its fast growing.
For forums, I tend to sell for about 8x my monthly earnings. More stable informational sites I do 12x my monthly earnings.
unfortunately your forum have no Pr , you have about 1050 Ylink as of today , but only from 16 domains , i am sure you are no where ranked in top 5 pages. given your money you make from adsens , you have some 60,000 pages viewed a month. Right now you are not better of selling the site. I will suggest wait and gain the PR up to 3 or 4 , then make more effort to get people to your website. you will see you will make more then upto $500 a month only from banner ads + at that moment you will get 10 times more then right now. i have a website for businesses for sale in Australia (http://www.business2sell.com.au), and the way we evaluate in industry right now is your business value = (total profit in last 24 months / 24) x 14 so if you your business makes $500 a month on average for 2 years. you will get 7000 $ for your website.
Sorry the prices given are totally unrealistic. They do not represent prospective sites, with a lot of potential. Which in this case, the sites deserve. Thanks anyway