It really depends on your personal situation and how much. If you're in the U.S., you'll have to check to see the min/max for suits in small claims court (this is generally anywhere from $3,000-$15,000). One thing that works, though, is merely having your lawyer draft a letter to the offending client. Your less intelligent clients will jump when they see any threat of legal action (and for you, it's only a few hundred bucks to get a lawyer to draft a letter – maybe even free depending on your relationship). There's certainly leg work involved, but when it's your livelihood, you have to do what you can.
