Average molecular weight of a DNA base pair

[Dr. Andrew Kropinski]

Help needed:
Many of the biotech. company catalogues (e.g. New England BioLabs, MBI 
Fermentas) include useful appendices which include as estimate of the 
molecular weight of single and double stranded DNA (sodium salts).  For 
double stranded DNA I have seen values of 649, 650 and 660 per bp. Does 
anyone have an original reference to these numbers, or can derive them 
from the average molecular weight of a base pair?
  Thankyou,
       Any ignorant microbiologist (Andrew Kropinski)