As a verb: I had to bargain with the shopkeeper.

As a noun: This thing that I bought was a bargain.

This is not an idiom because it means just what it says. If something is a bargain, then it is always a bargain. It doesn't matter what the circumstances are.

Sentences with the word bargain:

Noun: The secondhand table was a real bargain.

Verb: He bargained with the city council to rent the stadium.

No, bargain can not be an adverb.

Bargain

The plural of bargain is bargains.