Well a thoughtful question. And one with no single absolute answer.

Selling banner ads, monitoring them and following up with the customer can be a lot more time consuming then just operating an affiliate store or ad. Time is scarce. So I always have to figure that value of scarcity into my ad prices.

Sometimes it is not worth it.

Think of your websites as a large marble bathtub. As you add sites and traffic, your bathtub fills with water. As the water increases, the growing surface area allows for more pages and therefore places to run ads - either paid banners or affiliate program ads.

Steve