By Herbert Lash

NEW YORK, Oct 1 (Reuters) - The creation of a mammoth database that was ordered up in the wake of the Flash Crash will cost $255.6 million over five years to operate, according to the average cost estimate of six entities vying to build the system, a regulatory filing showed.

The database and surveillance system, dubbed the consolidated audit trail (CAT), will cost on average $53 million to build, the bidders estimated in a 166-page document that was published Wednesday by the Securities and Exchange Commission.

The CAT, one of Wall Street's biggest undertakings in years, took on urgency after the Flash Crash when it took months for the SEC to reconstruct trading on May 6, 2010 when the Dow plunged almost 10 percent in minutes and then mostly recovered.

The document outlines the formation of a limited liability company to be called CAT NMS LLC, which will build, implement and operate the database and audit trail after the SEC's approval following a period of public comment.

Eighteen U.S. stock and options exchanges, and brokerage watchdog the Financial Industry Regulatory Authority, prepared the document after almost two years of study, including 36 meetings with an advisory committee of industry participants.

The exchanges and FINRA, which also is one of the six bidders, will govern the limited liability company after the winning bidder is selected. The winner will be chosen within two months of the SEC's approval of the plan to roll out the CAT.

The database, which will track and store 58 billion records of orders, executions and quotations for equities and options markets on a daily basis, will be the largest in the financial services industry, topped only by U.S. government entities.

(Reporting by Herbert Lash; Editing by Kim Coghill)