The Collaborative International Dictionary of English v.0.48:

Prorate \Pro*rate"\, v. t. [imp. & p. p. Prorated; p. pr. &
   vb. n. Prorating.] [From L. pro rata (sc. parte) according
   to a certain part, in proportion.]
   To divide or distribute proportionally; to assess pro rata.
   [U.S.]
   [1913 Webster]