No one else has responded, so I'll try. Dewar's white is not a particularly wonderful scotch i my humble, but a nice decanter is a nice decanter. Are you sure it's white label and not one of the better dewars products?
After speaking to the Dewars compamy archivist in Glasgow I am told that unless you are getting the bottle in original packaging and in immaculate condition it is not a huge bargain.
The flagons are still quite common and go for around £45-50($72-80US)
Hope this has been a help.