Price is not the only value that you can get from a professional in travel. When you talk to an agent that specializes in diving and have travelled to the destination that you are thinking on visiting he or she can help you decide where to stay, what to do, what you should avoid, what are the must do in the area, all this really help you in getting the most of your time and money while in a new place. So you won't only get the best out of your vacation but at the end you will also save money and time, and in the worst case scenario you will be paying the same for your vacation that you will pay on any of the big website but with no professional advice at all, not many dive instructors/agents and with thousands of dives aboard work for one of those, right?