What is the cheapest 5.56 rifle?

Answer 1

This is a difficult question to answer, as it is so vague. It will be answered on the assumption that the question is referring to a semi auto rifle with a 5.56 chamber (as not all .223 rifles are capable of firing military 5.56 ammunition).

As far as brand new rifles would go, for the US market, your answer would likely be the Saiga .223, variations of which can be found in the $350 - $400 price range, and are capable of chambering military 5.56 ammunition.