The Collaborative International Dictionary of English v.0.48:

Cheapen \Cheap"en\, v. t. [imp. & p. p. Cheapened; p. pr. &
   vb. n. Cheapening.] [OE. cheapien, chepen, to trade, buy,
   sell, AS. ce['a]pian; akin to D. koopen to buy, G. kaufen,
   Icel. kaupa, Goth. kaup[=o]n to trade. Cf. Chap to
   bargain.]
   1. To ask the price of; to bid, bargain, or chaffer for.
      [Obsoles.]
      [1913 Webster]

            Pretend to cheapen goods, but nothing buy. --Swift.
      [1913 Webster]

   2. [Cf. Cheap, a.] To beat down the price of; to lessen the
      value of; to depreciate. --Pope.
      [1913 Webster]

            My proffered love has cheapened me.   --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cheapen
    v 1: lower the grade of something; reduce its worth [syn:
         degrade, cheapen]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "cheapen":
   adulterate, alloy, bargain, beat down, bid, bid for, break, canker,
   chaffer, coarsen, confound, contaminate, corrupt, cut, cut prices,
   debase, debauch, decline, decry, defile, deflate, deflower,
   degenerate, degrade, denature, deprave, depreciate, desecrate,
   despoil, devaluate, devalue, dicker, distort, dive, downgrade,
   drive a bargain, fall, fall in price, give way, haggle, higgle,
   huckster, infect, jew down, lower, mark down, misuse, negotiate,
   nose-dive, outbid, pare, pervert, plummet, plunge, poison, pollute,
   prostitute, ravage, ravish, reduce, sag, shave, slash, slump,
   taint, trim, twist, ulcerate, underbid, undervalue, violate,
   vitiate, vulgarize, warp, write down, write off