When searching for a couples counsellor, it is important to consider a variety of factors:
Qualifications: education, credentials, certifications, licenses and professional memberships.
Areas of specialty. It is not uncommon for counsellors to have several speciality areas. Be sure couples counselling is one of them.
Experience. As with any profession, the more experienced a counsellor is, you may find they can be slightly more expensive. On the other hand, new counsellors who are just starting out in their practice may charge less than a counsellor with 5 or 10 years of experience. While you may save money initially, it can take months if not years to effectively transfer theory taught in classrooms into practical application with a couple.
Approach to Therapy. As I have indicated here Emotionally Focused Therapy has substantial research indicating it is the most effective therapy for couples counselling.