Same on all my sites, link units pay almost nothing. I don't get it. People actually have to click twice, risk for accidental clicks is extremely small. If anything they should pay more. On one my sites I had 1800 clicks on a link unit in November and earned 24 dollars.
I guess that link units are not as descriptive as to what they are. Other bigger ads are a lot better for advertisers. You also may be experiencing smaller pay because you have other bigger ads around your site so the lower paying advertisers are being put into the link units.
Actually link-units don't do badly for me at all. I find it really depends on where you put them though. Above the fold works best.
Last month link units gave me 9% of my earnings. There is a tradeoff between the money any ad can generate and how visible it is on your site. Link units can be placed in a prominent position on every page of a site without using up too much space, or creating an eyesore. So they do earn less but they also have less impact on each page than, say, a leaderboard.
Yea, if I have a page that is under performing (like a non-technical page with a CTR of less than 2%) I add a link unit below the title. Usually the ad-description will actually be interesting to my visitors, but if they want to explore my site, they can easily see what it's about as well. I'm very careful to put ads up in a way that doesn't annoy visitors too much. They are after all the primary reason the site's there in the first place. Link units help with that.