Some food for thought....
First you should exhaust every friendly collection remedy you have, such as discounting the total amount due, making sure the invoices are in the correct hands of the decision makers/bill payers (sometimes a friendly letter to the CEO will do wonders), or otherwise get them to tell you in writing theyre not paying you. Make sure they are not witholding payment due to being unhappy with your service or product.
If that fails, consider a collection agency. Remember though, a collection agency often times will negotiate a payment for a lesser amount (something you can try yourself), so if they get the client to pay 75% of the invoice then the collection agency will zap another 25% of that, thus leaving you with only 50% of the face value. If the collection agency will purchase the invoice, maybe consider that option as well.
Maybe throw a little money at a lawyer to write a demand letter to get their attention.
If the client still refuses and you go with a collection agency, be sure youre with an agency that will sue to recover, especially if the client is out if state. They can add attorney fees in if your contract has an attorney fees provision. If verbal contract only, then you would bear your own legal expenses.
If the client is out of state, you cant use small claims. In small claims, state law prevents companies or individuals from being served out of state, except under limited circumstances (none of which apply here). The next court level is limited jurisdiction, where all the court formalities apply and its difficult to maintain a lawsuit in pro per.
If the client is within the state, small claims might be a viable and quick option. As others have said, two cases for $5K each. You lose $4K, but then again, you might lose that amount anyway if you go the collection route. Write the balance off as a loss.
If the client is here in Cali and you get a judgment I can help you collect it. Ill PM you the name of a good collection lawyer who is a client of mine.