But, Yuin PK3 is an ear bud which would offer no isolation. I thought he was asking about IEMs mainly going by the mention of M2 in his query.
Even in that case, I'd recommend Blox M2C if it gets back in stock (
http://www.bloxearphone.com/). It is right now said to be the best ear bud under $50. Am likely to get one later on.