What you're referring to is a demand letter. That would be the first step in the collection process. The cost would depend on an attorney. Some attorneys would do it on hourly basis. Some might do it for a flat fee. A demand letter may lead to negotiations and an arrangement for payment. If not, you may have to file a complaint and get a judgment against the business. You are welcome to call for a fee quote.
Please note that the above is not intended as legal advice, it is for educational purposes only. No attorney-client relationship is created or is or is intended to be created by my answer. You should contact an attorney in your area to discuss your case.