The Collaborative International Dictionary of English v.0.48:

Highway robbery \High"way` rob"ber*y\, n.
   1. Robbery committed on the public roads.
      [PJC]

   2. An excessively high price or fee; -- used especially in
      situations where the buyer has little or no choice but to
      buy the item offered.
      [PJC]

WordNet (r) 3.0 (2006):

highway robbery
    n 1: an exorbitant price; "what they are asking for gas these
         days is highway robbery"
    2: robbery of travellers on or near a public road

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "highway robbery":
   armed robbery, asportation, assault and robbery, banditry,
   bank robbery, cattle lifting, cattle stealing, exploitation,
   extortion, heist, hijacking, holdup, loan-sharking, mugging,
   overassessment, overcharge, pocket picking, purse snatching,
   robbery, robbing, shylocking, stickup, stickup job, surcharge,
   usury