"There seems to be ( in general ) two groups of knife owners, those who use them as a tool for their job ie.Tradesmen, servicemen etc. and those who collect knives as a hobby for their beauty, craftsmanship, resale value etc."
I think this quote is a good base for my observation.  The question of what a knife is worth is a question of value.  Ultimately, the knife's value will be decided by what the individual values in that knife.  If the knife is meant to look pretty, a customized Sebenza will, IMHO, be a more valuable purchase.  If the knife is to fill a general working tool role, an SAK will prove to be a better purchase.  So far, I've just summarized a good bit of the other posts...  

   Now, onto the rest of my thought... 
I would add a third class of knife users to the original two listed - a hybrid class, among which I count myself.  The hybrid user carries a knife for life's possibilities, whether they be mundane tasks like opening the mail or extreme circumstances, like cutting someone out of their seatbelt, freeing them from a burning car and grisly end.  The hybrid user likes a knife to be both aescetically pleasing and functionally durable.  Now, I believe that most people from the three groups of knife users have these same values.  What makes the hybrid user unique is that he/she does not necessarily value one quality over the other.  To the hybrid, form and function are equally valuable.
With that in mind, I think that expensive can mean better.  But, as others have pointed out, the user/purchaser hits a ceiling in the $100-$150 range.