It depends largely on the counselor, the setting, and the people involved. The outcome is similar to making a cake...you can get red velvet or chocolate depending on the ingredients involved. I enjoyed it because we had a GREAT counselor, and we were both honest with her so it was productive.
The cost varies as much as the counselors do. Ours was $90/hour.
What you do also varies widely...some folks just talk, some have you do exercises, some discuss childhood, sometimes you answer questionnaires, other times you address issues with the other, discuss perceptions of marriage, problems, etc. Some counselors have a set curriculum that they use for premarital situations while others address specific needs of the couple.
I VERY, VERY, VERY, VERY, VERY, VERY highly recommend it. There is, IMO, nothing more important. The ring, the chapel, and the invitations can wait. Premarital counseling is an INVESTMENT in the marriage...or a sobering revelation of the reasons there shouldn't be one...
HTH