The Collaborative International Dictionary of English v.0.48:

Reasonably \Rea"son*a*bly\, adv.
   1. In a reasonable manner.
      [1913 Webster]

   2. Moderately; tolerably. "Reasonably perfect in the
      language." --Holder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reasonably
    adv 1: to a moderately sufficient extent or degree; "pretty
           big"; "pretty bad"; "jolly decent of him"; "the shoes are
           priced reasonably"; "he is fairly clever with computers"
           [syn: reasonably, moderately, pretty, jolly,
           somewhat, fairly, middling, passably] [ant:
           immoderately, unreasonably]
    2: with good sense or in a reasonable or intelligent manner; "he
       acted sensibly in the crisis"; "speak more sanely about these
       affairs"; "acted quite reasonably" [syn: sanely,
       sensibly, reasonably] [ant: unreasonably]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "reasonably":
   analytically, artfully, astutely, at a bargain, at a sacrifice,
   at budget prices, at cost, at cost price, at piggy-bank prices,
   at prime cost, at reduced rates, at small cost, at wholesale,
   cheap, cheaply, circumspectly, cleverly, considerately, cunningly,
   discerningly, discreetly, for a song, for pennies,
   in all conscience, in all reason, in reason, inexpensively,
   intelligently, judiciously, knowingly, logically, moderately,
   nominally, on the cheap, providently, prudently, rationally,
   sagaciously, sanely, sensibly, shrewdly, smartly, soundly,
   syllogistically, thoughtfully, understandingly, wholesale, wisely,
   within bounds, within reason, within reasonable limitations