What is a consultant worth? Short answer: Whatever the customer agrees to pay! (I'm a capitalist at heart!)
But that is a wide-open question. Consulting is governed by supply-and-demand as much as anything else -- there are other factors, but S&D is certainly a biggie. While there may be lots of "ISO-9001" consultants out there, there are fewer consultants with expertise, say, in designing space telescopes, or making synthetic diamonds, thus their per-day rates will be much higher.
In the quality tools, quality system, and general "business" training (supervisory training, conflict management, safety, etc.) categories I've seen a day rate range of less than $200/person for attendance at a community college training seminar to $1000/day for an on-site ISO consultant.
In the final analysis, some consultants were "worth" every penny paid plus some, and some weren't worth the waste in personnel time expended even if they had come for free.