Does anyone know the actual value of a single ad impression?
Some adverts are pay per view, pay per click, some are commission based (referrer gets a cut). Sometimes it'll be a few dollars for a few thousand views, other times it'll be a few hundred dollars for a week's visibility.
For click throughs, fractions of a cent to a few cents I guess. Commission is obviously worth a lot more (a few percent of the purchase price typically), but harder to convert users.
Have a look at buysellads to get an idea of how this works. Unsurprisingly Adblock has a field day if you try and view the website with it turned on ;)
This is often summarized as RPM (revenue per mille) or eRPM (effective RPM), i.e. how much income a site's ads bring in per 1000 page impressions, regardless of how it's paid out (per-click, per-view, per affiliate purchase, etc.). And that varies hugely between sites, anywhere from below $1 to above $100. A typical case is around $5-10 for many publishers, I believe. So that's about 0.5-1.0¢ per page view.
Subscriptions do work for some sites, but from what I can tell most have trouble selling enough subscriptions to cover what advertising would pay for.