A massive, failed deal that keeps paying out money to Wall Street

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A relic of failure from the buyout boom is the gift that keeps on giving to Wall Street banks and law firms.

It's the deal that keeps on giving — to banks and law firms, at least.

Undoing the leveraged buyout of Texas utility TXU, a botched $45 billion deal that's now nearly a decade old , keeps paying out the advisors that assembled the massive, failed transaction. They may have bagged $1 billion on the saga already, according to a mergers and acquisitions expert.

Investment banks' "transactional fees since 2008 have been about $625 million" on the TXU deal, said Freeman & Co.'s Jeffrey Nassof, who tracks M&A fees for big banks. "I've seen estimates that the legal and restructuring advisory fees could be several hundred million dollars. Adding that to [bankers'] fees could get you to the $1 billion range."

The latest chapter in the saga of TXU, which was renamed Energy Future Holdings, is paying big banks once again.

On Friday, when NextEra Energy announced it would buy Energy Future Holdings and its stake in Oncor Electric Delivery in a deal worth $18.4 billion, it meant bankers from Credit Suisse, Bank of America Merrill Lynch , Deutsche Bank , JPMorgan Chase , UBS and Wells Fargo racked up a payday for Wall Street. The banks were not immediately available to comment.

This time around, the banks that advised NextEra and Oncor would likely have up to $75 million in fees to divvy up, once the deal closes, Nassof said. But that's just a drop in the bucket compared with what Wall Street has reaped on the massively complex buyout.

It's been nearly 10 years since one of the biggest deals in the history of the private equity business went down, and more than two years since Energy Future Holdings filed Chapter 11 bankruptcy , costing investors from Henry Kravis to Warren Buffett billions in the process. A Buffett representative did not respond to a request for comment; private equity firm KKR , which Kravis continues to lead, declined to comment.

And there's still more money to be made: Energy Future Holdings still has to exit Chapter 11, after its lengthy stay, meaning advisors will be getting more work in the future.

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