I picked up a slightly used size extra-small for a friends daughter in the National Guard last year for $150. The full size level IV plates run about $400 extra. Every U.S. serviceman should be issued a set with their first BDU.
Not much more than the old non-ballistic flak jackets. They are quite a bit stiffer, but not as bulky. The plates do have a considerable heft to them, but less than the old issue chicken plate.