Every webmaster is complaining about adsense earnings nowadays. Some are saying about economic crisis, but….
Adsense was much more profitable a few years ago. On my retail site I used to make monthly what it takes 3 months now and traffic has increased over the years. I think Google is not paying what it once did. I noticed the drop in revs when they went to bidding on (Adword) keywords.
I think it depends on what affiliate programs are on the site, some retail programs only pay 8% and Adsense would pay more in most cases.
Also it is doubtful someone would earn more with Adsense than what (gambling) affiliate programs earn. However, the good think about Adsense earnings (compared to gambling) is they’re just sent on time every time. No song and dance and no chasing.