Some litigators won't send a demand letter like that without the client being willing to go farther if the former employer doesn't cease and desist, because they don't want to get a reputation for making litigation threats that they don't then back up with action.
If you do find one who will send a letter, I'd guess 2 hours of work, 1 hour to meet with you and discuss the situation and review any important documents, and a 2nd to write the letter.
Lawyers' hourly rates vary in Southern California, from $200-400 per hour.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
