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]

WordNet (r) 3.0 (2006):

prorate
    v 1: make a proportional settlement or distribution
    2: divide or assess proportionally; "The rent was prorated for
       the rest of the month"

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "prorate":
   ask, assess, charge, charge for, demand, divide, divvy, exact,
   impose, levy, make dutiable, parcel, portion, pro rata, proportion,
   proportionate, quota, ration, require, share, shift, stick for,
   tax, tithe