Does anyone want to actually answer the question?
AFAIK, it is legal for ordinary citizens to buy body armor. Cost depends on the extent of coverage and the threat level.
Check local and state law, though. Some states ban possession of body armor by convicted felons or its use in commission of a crime. I have heard, but not confirmed, that a few cities ban it on the usual grounds that civilians should never be allowed to defend themselves. This is known as the "if you are killed, call the police" theory of protect and serve.
Jim