To the guy talking about Adsense conversions above.
More than anything -- including # of unique visitors -- does it matter where in the "funnel" your keywords are. I have sites with 100-270 UVs a day which are making me good x,xxx$ a month (not Adsense). Those are targeting KWs which are deep down the funnel -- buyers, not tire kickers.
A stupid example from the top of my head: think about the keywords: "bike" > "bike rental" > "best bike rental Barceloneta" > "{service name} bike rental barcelona review" ... The latter two have fewer searches, but they are specific. Such specific phrases are made by buyers, ready to throw money a specific service/product, or at least someone considering doing it if you can convince them with copy-writing magic.
This funnel concept is reflected in everything from number of opt-ins (if you're building a list), CPC on adsense, %conversions, CTR .... I don't really do Adsense, but I still have some news/viral sites running these ad-blocks. These sites post videos, funny pics and other stuff people tend to share. The "general" traffic these sites pull, leads to a low CPC, low CTR --even with thousands of UVs -- and make me less than my product sites, despite having 10x+ the number of UVS.
It makes sense, as general traffic converts less for the advertisers, and thus they're unwilling to pay the same for these terms as for buyer kws. You can see this if you're running CPAs, or pushing products from Clickbank. Here you are both the publisher and the advertiser -- sort of. Naturally, you can see how your ads convert in % of UVs at your site, but you can also see how the prospects convert on the advertisers landing page.
One of my above sites have a 1:8--1:10 conversion on SEO traffic (depending on day of month, season and other force majeure), and converts in 1:8 hops on the advertiser site. Basically I hit 100$ a day when I have above 16--20 hops a day. I've ranked other more general sites for the same product, and it's not near these numbers. Naturally, if I were to buy CPM/CPC I would pay more for having this targeted traffic.