The Collaborative International Dictionary of English v.0.48:

bunce \bunce\ n.
   a sudden unexpected piece of good fortune.

   Syn: windfall, gravy, godsend.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bunce
    n 1: a sudden happening that brings good fortune (as a sudden
         opportunity to make money); "the demand for testing has
         created a boom for those unregulated laboratories where
         boxes of specimen jars are processed like an assembly line"
         [syn: boom, bonanza, gold rush, gravy, godsend,
         manna from heaven, windfall, bunce]