Source : Webster's Revised Unabridged Dictionary (1913)

Overvaluation \O"ver*val`u*a"tion\, n.
   Excessive valuation; overestimate.

Source : WordNet®

overvaluation
     n 1: an appraisal that is too high [syn: {overestimate}, {overestimation},
           {overappraisal}]
     2: too high a value or price assigned to something