I work logistics and have the pleasure of working with carriers on this sort of stuff. It's absolutely variable. The companies essentially go to UPS and/or Fedex, explain how much volume they'll be doing, and the UPS/Fedex rep goes to their pricing department and tries to plead their case to get rates as low as possible for their customer. The big boys have more business to flaunt, therefore, get better rates.
Hazmat charges won't be quite as fluctuating as regular pricing on standard shipping just due to the extra training, handling, etc., but it is still variable to some degree.
What I do, and I'm sure these companies do, is essentially get UPS and Fedex in a bidding war for their business, which will drive down costs.