While it varies from case to case, have you considered running affiliate offers instead of just regular banner ads? Might be worth testing - if you could get banners up advertising the next big game, you can earn a commission if they pre-release through your ad. I've found that can often earn more than regular display ads, but might be worth testing. (By the way, while Amazon converts well, it only has a cookie length of 1 day, while most others have a cookie length of around 30 days, so you could also try testing different affiliate merchants).