The Collaborative International Dictionary of English v.0.48:

Priceless \Price"less\, a.
   1. Too valuable to admit of being appraised; of inestimable
      worth; invaluable.
      [1913 Webster]

   2. Of no value; worthless. [R.] --J. Barlow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

priceless
    adj 1: having incalculable monetary, intellectual, or spiritual
           worth [syn: invaluable, priceless]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "priceless":
   absurd, amusing, beyond price, bizarre, cherished, comical, costly,
   dear, droll, eccentric, expensive, funny, golden, good as gold,
   high-priced, hilarious, humorous, hysterical, incalculable,
   incongruous, inestimable, invaluable, irreplaceable, laughable,
   ludicrous, of great price, precious, prized, quaint, quizzical,
   rich, ridiculous, riotous, risible, screaming, treasured, unique,
   valuable, valued, whimsical, witty, worthy