"
 From http://history.nasa.gov/SP-4221/ch9.htm: 
An accompanying table compared costs for five
options:

                  CASE
               1
               2
               2A
               3
               4
      
                  .
      
                  Payload bay (ft.)
               10 x 30
               12 x 40
               14 x 45
               14 x 50
               15 x 60
      
                  Payload weight (lbs)
               30,000
               30,000
               45,000
               65,000
               65,000
      
                  Development cost ($billions)
               4.7
               4.9
               5.0
               5.2
               5.5
      
                  Operating cost
         ($millions/flight
               6.6
               7.0
               7.5
               7.6
               7.7
      
                  Payload costs ($/pound)
               220
               223
               167
               115
               118
      

It appears that this figure came from Case 5, which is what was built.  "