The Serious Fraud Office spent $1,105,111 investigating Hanover Finance.
Investors were owed about $550 million when Hanover suspended payments nearly five years ago.
The SFO spent three years investigating the firm, but announced it would not be taking legal action on 30 April this year.
It says its own staff spent 12,700 hours on the case, clocking up about $600,000, while contractors billed $505,111.
SFO acting chief executive Simon McArley says it is important to do these types of investigations to make sure that nothing illegal has taken place.
Mr McArley says he shares investors' disappointment that there wasn't a more clear-cut outcome.