Some of the costs are easy to see, like the acute embarrassment some experience from an intimate "pat down" if they elect to avoid any risk from x-ray radiation.
Others are much more difficult, such as potential health consequences of the x-ray radiation exposure. Since the TSA has refused to disclose the x-ray flux, it is hard to even get started. The comparisons the TSA makes with other radiation sources are flawed because they assume the dose is uniform throughout your body when it is actually highest at the surface.
But even if an outside evaluator did have good numbers to start with, in the end the best you can expect are extrapolations using data from dramatically different radiation exposure, which is the nature of calculating radiation risks. The only way to have a non-extrapolated number is to expose half of a sample population to the scanner radiation and measure outcomes for 50 years.
Avoided consequences are easier to calculate. Of the 8,000,000,000 or so US domestic air travelers of the past ten years, zero have smuggled a bomb on board, and zero have attempted to. Zero out of billions of passengers.
The worst consequences are the loss of an aircraft. A terrorist cannot take control of an aircraft using a bomb, only destroy it. An X-ray scanner will not stop someone from bringing a knife onboard (even if it could, it is probably not possible to take control of an aircraft with a knife due to cockpit doors, passenger awareness, etc.)
So, cost benefit sum-up: zero attempts to smuggle bomb onto domestic flight over past 10 years, unknown but likely low health risk, consequences limited to loss of one aircraft due to screening failure.
At least two have succeeded in smuggling a bomb on board, but failed to detonate them. Look up the "Christmas bomber" and the "shoe bomber".