The Collaborative International Dictionary of English v.0.48:

Extravagance \Ex*trav"a*gance\, n. [Cf. F. extravagance. See
   Extravagant, and cf. Extravaganza.]
   1. A wandering beyond proper limits; an excursion or sally
      from the usual way, course, or limit.
      [1913 Webster]

   2. The state of being extravagant, wild, or prodigal beyond
      bounds of propriety or duty; want of moderation; excess;
      especially, undue expenditure of money; vaid and
      superfluous expense; prodigality; as, extravagance of
      anger, love, expression, imagination, demands.
      [1913 Webster]

            Some verses of my own, Maximin and Almanzor, cry
            vengeance on me for their extravagance. --Dryden.
      [1913 Webster]

            The income of three dukes was enough to supply her
            extravagance.                         --Arbuthnot.

   Syn: Wildness; irregularity; excess; prodigality; profusion;
        waste; lavishness; unreasonableness; recklessness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

extravagance
    n 1: the quality of exceeding the appropriate limits of decorum
         or probability or truth; "we were surprised by the
         extravagance of his description" [syn: extravagance,
         extravagancy]
    2: the trait of spending extravagantly [syn: extravagance,
       prodigality, profligacy]
    3: excessive spending [syn: extravagance, prodigality,
       lavishness, highlife, high life]

Moby Thesaurus II by Grady Ward, 1.0:

300 Moby Thesaurus words for "extravagance":
   abandon, absurdity, abundance, affluence, aggrandizement, amenity,
   ample sufficiency, ampleness, amplification, amplitude, avalanche,
   ballyhoo, bedizenment, big talk, bigotry, blatancy, blowing up,
   bonanza, boundlessness, bountifulness, bountiousness, brazenness,
   bumper crop, burlesque, capriciousness, caricature, carpe diem,
   cloud of words, colorfulness, copiousness, crapulence, crapulency,
   crapulousness, crudeness, dash, dazzle, dazzlingness, deluge,
   diffuseness, diffusion, diffusiveness, dilatation, dilation,
   dissipation, drunkenness, duplication, duplication of effort,
   effusion, effusiveness, egregiousness, embarras de richesses,
   embellishment, enhancement, enlargement, enormousness, enough,
   exaggerating, exaggeration, excess, excessiveness, exorbitance,
   exorbitancy, expansion, expletive, extortionateness, extravagancy,
   extravaganza, extravagation, extreme, extremeness, extremes,
   extremism, extremity, exuberance, fabulousness, fanaticalness,
   fanaticism, fantasy, fat, featherbedding, fecundity, fertility,
   filling, flagrancy, flamboyance, flashiness, flightiness, flood,
   flow, fluency, foison, formlessness, frill, frills, frippery,
   full measure, fullness, gaiety, garishness, gaudery, gaudiness,
   generosity, generousness, giantism, gigantism, gingerbread, glare,
   glitter, gluttony, gorgeousness, grandiloquence, great abundance,
   great plenty, gush, gushing, heightening, huckstering, hyperbole,
   hyperbolism, hypertrophy, immoderacy, immoderateness, immoderation,
   improvidence, incontinence, indiscipline, indulgence, inflation,
   inordinacy, inordinance, inordinateness, intemperance,
   intemperateness, inundation, irrationality, jauntiness, jazziness,
   landslide, lavishness, leaking purse, liberality, liberalness,
   logorrhea, loose purse strings, lots, loudness, luridness,
   luxuriance, luxury, macrology, magnification, maximum,
   meretriciousness, money to burn, monstrousness, more than enough,
   much, myriad, myriads, needlessness, nimiety, numerousness,
   obtrusiveness, opulence, opulency, ornamentation, outpour,
   outpouring, outrageousness, overabundance, overaccumulation,
   overadornment, overbounteousness, overcopiousness, overdevelopment,
   overdoing, overdose, overemphasis, overenthusiasm, overestimation,
   overflow, overgenerosity, overgenerousness, overgreatness,
   overgrowth, overindulgence, overkill, overlap, overlargeness,
   overlavishness, overliberality, overluxuriance, overmeasure,
   overmuch, overmuchness, overnumerousness, overplentifulness,
   overplenty, overpopulation, overprofusion, overreaction,
   overreligiousness, overstatement, oversufficiency, oversupply,
   overzealousness, padding, palilogy, panache, payroll padding,
   perfervidness, plenitude, plenteousness, plentifulness, plenty,
   pleonasm, plethora, pound-foolishness, preposterousness,
   prevalence, prodigality, productiveness, productivity, profligacy,
   profuseness, profusion, prolificacy, prolificity, prolixity,
   puffery, puffing up, quantities, rabidness, radicalism, rampancy,
   rankness, reckless expenditure, reckless spending, recklessness,
   redundance, redundancy, reiteration, reiterativeness,
   repetition for effect, repetitiveness, repleteness, repletion,
   rich harvest, rich vein, richness, riot, riotousness, scads,
   self-indulgence, sensationalism, shamelessness, shower, showiness,
   spate, sportiness, squander, squandering, squandermania, stream,
   stretching, substantiality, substantialness, superabundance,
   superfluity, superfluousness, superflux, superlative, swinishness,
   talkativeness, tall talk, tautology, tawdriness, teemingness,
   tirade, too much, too-muchness, touting, travesty,
   ultrazealousness, unconscionableness, unconstraint, uncontrol,
   undueness, unnecessariness, unreasonableness, unrestrainedness,
   unrestraint, verbosity, vulgarness, waste, wastefulness, wealth,
   whim, zealotism, zealotry