Here are some numbers to work with . Now divide those print CPM costs by 4 to adjust for pass-along reach and you have a basis for comparison. Getting a site to tell you how many individuals they reach may be a problem for obvious self-serving reasons. But it's individuals not eyeball impacts who buy, so you need to know. Then make downward adjustments to that price for the other factors I discuss below.
Banners are about as useful as bumper stickers for branding and building mind share.
It's not branding, it's bunk. But at least one well known site tried that then went back to billing for every visitor to the page, whether your ad is shown or not. Their justification is that some visitors will see the ad because their browser cached it, and any new visitors will see the ALT copy and a single line of text below the rectangle. I think if I'm there with my graphics off now, they were probably off the last time, too, so there's nothing in the cache. Larger ads make more of an impression, have more impact and higher readership, and therefore do you more good than smaller ads.