"Talk is cheap until you hire a lawyer." Can someone please explain to me how this is an example of punch line?

That is called "satire".
Anyone can talk and talk all they want to, no cost! just energy and air is all that is required. <G> BUT, when a lawyer is involved, then some people say, one pays by the breath.... so talk isn't cheap at all.

Here is a good site for defining satire:
http://www.answers.com/topic/satire
http://dictionary.reference.com/browse/satire