Source : Webster's Revised Unabridged Dictionary (1913)

Overpay \O`ver*pay"\, v. t. [imp. & p. p. {Overpaid}; p. pr. &
   vb. n. {Overpaying}.]
   To pay too much to; to reward too highly.

Source : WordNet®

overpay
     v : pay too much [ant: {underpay}]
     [also: {overpaid}]