The Security Mentor

This is about hiring a consultant in general, not specifically about hiring a security consultant, but it has good advice about how to check for conflicts of interest and what benefits you get for your money:
Why Your Business Needs an IT Consultant

Even if you have enough expertise in house to keep your system secure from day to day, it can make sense to hire an outsider periodically to give you a checkup. Another good reason would be installing new and unfamiliar equipment, or spinning up new Internet services. You might also want a security consultant if you are getting hit by new government regulations and need someone to translate them into concrete actions.