The Collaborative International Dictionary of English v.0.48:

Reasonableness \Rea"son*a*ble*ness\, n.
   Quality of being reasonable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

reasonableness
    n 1: the state of having good sense and sound judgment; "his
         rationality may have been impaired"; "he had to rely less
         on reason than on rousing their emotions" [syn:
         rationality, reason, reasonableness]
    2: goodness of reason and judgment; "the judiciary is built on
       the reasonableness of judges"
    3: the property of being moderate in price or expenditures; "the
       store is famous for the reasonableness of its prices"; "the
       modestness of the living standards here becomes obvious
       immediately" [syn: reasonableness, moderateness,
       modestness]
    4: moderation in expectations; "without greater reasonableness
       by both parties we will never settle this matter!"
    5: the quality of being plausible or acceptable to a reasonable
       person; "he questioned the tenability of my claims" [syn:
       reasonableness, tenability, tenableness]