Read this very motivational thread, if you are interested in buying an existing domain name and optimizing the web traffic there:
Some juicy quotes:
"I paid about 16-18 months profits for each of these sites based on their previous owners' earnings. "
"For both sites I basically just increased the size of the ad units being used. I added a network to the ipod site which brought 400 or so a month. The counterstrike site was only running adsense and I knew from previous experience if I added display ads (like banners) I could double the income."
Happy Domain Hunting!