A physics paper has been published (PDF of paper) on their radiation and effectiveness.
Their conclusions are that the radiation does penetrate into the body--it uses a CT scan level of radiation--and the machines are easy to pass since spatial contrast to the body is what makes objects stand out.
That is, a "pancake" type object would probably pass through as opposed to a "brick" shape. They also suggest metal wires are small enough to pass as well. For that matter, they point out stuff hidden on one's sides, like a small gun, would probably pass through since it wouldn't be in front of anything.
This website says each machine costs between $100-200k and the Dept. of Homeland Security has put in a request for $72M for them.