Andy L , Jun 30, 2009; 10:13 a.m.
The cheapest? Look at the Sigma 70-300 but make sure it's the newer version with the built in motor. Their model naming is so bad the guys at Calumet couldn't tell me exactly what I was holding until I mounted it on a camera, tried the AF and listened for what it sounded like. It's also got some usefulness as a macro lens.
But also do look at the Nikon 55-200 AF-S VR. It's reasonably priced and quite good as "kit" type lenses go.