tidy sum

noun

A large sum of money: fortune, mint. Informal bundle, pretty penny, wad. Slang pile. See rich/poor.

The noun has one meaning:

Meaning #1: (often followed by `of') a large number or amount or extent
  Synonyms: batch, deal, flock, good deal, great deal, hatful, heap, lot, mass, mess, mickle, mint, muckle, peck, pile, plenty, pot, quite a little, raft, sight, slew, spate, stack, wad, whole lot, whole slew