The Collaborative International Dictionary of English v.0.48:

Exorbitant \Ex*or"bi*tant\, a. [L. exorbitans, -antis, p. pr. of
   exorbitare to go out of the track; ex out + orbita track: cf.
   F. exorbitant. See Orbit.]
   1. Departing from an orbit or usual track; hence, deviating
      from the usual or due course; going beyond the appointed
      rules or established limits of right or propriety;
      excessive; extravagant; enormous; inordinate; as,
      exorbitant appetites and passions; exorbitant charges,
      demands, or claims.
      [1913 Webster]

            Foul exorbitant desires.              --Milton.
      [1913 Webster]

   2. Not comprehended in a settled rule or method; anomalous.
      [1913 Webster]

            The Jews . . . [were] inured with causes exorbitant,
            and such as their laws had not provided for.
                                                  --Hooker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

exorbitant
    adj 1: greatly exceeding bounds of reason or moderation;
           "exorbitant rent"; "extortionate prices"; "spends an
           outrageous amount on entertainment"; "usurious interest
           rate"; "unconscionable spending" [syn: exorbitant,
           extortionate, outrageous, steep, unconscionable,
           usurious]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "exorbitant":
   a bit much, abandoned, acute, aggrandized, amplified, ballyhooed,
   boundless, clamant, clamorous, crying, cutthroat, cutting,
   demanding, disproportionate, dizzy, draining, drastic, egregious,
   enormous, exacting, exaggerated, excessive, exigent, extortionate,
   extraordinary, extravagant, extreme, fabulous, fancy, fierce,
   furious, gigantic, gluttonous, gouging, grandiloquent, grasping,
   great, grossly overpriced, high, high-flown, hyperbolic,
   hypertrophied, immoderate, importunate, incontinent, inflated,
   inflationary, inordinate, insistent, instant, intemperate, intense,
   keen, loud, magnified, monstrous, out of bounds, out of sight,
   outrageous, overbig, overboard, overdeveloped, overdone, overdrawn,
   overemphasized, overemphatic, overestimated, overgreat, overgrown,
   overlarge, overmuch, overpraised, overpriced, oversold, overstated,
   overstressed, overweening, overwrought, persistent, pertinacious,
   piercing, preposterous, pressing, prodigal, profuse, prohibitive,
   puffed, rigorous, rough, severe, sharp, skyrocketing, spiraling,
   splitting, steep, stiff, stretched, superlative, taxing, too much,
   tough, touted, towering, unbridled, unconscionable, undue,
   unjustifiable, unjustified, unmeasurable, unreasonable,
   unrestrained, unwarranted, urgent, usurious, vehement, venomous,
   violent, virulent