Department changes were explained only in broad terms, a report says
The government spent £780m reorganising its departments and agencies in the four years after the 2005 election, Whitehall's spending watchdog says.
The National Audit Office says more than 90 such moves happened, some of which may have been unnecessary.
Detail of the value for money achieved was "unsatisfactory", its report adds.
But the government said this was a "key priority", adding that it had to be able to act quickly to make reforms where they were needed.
The National Audit Office said it was impossible to demonstrate that the changes made between the May 2005 general election and June 2009 represented value for money.
Most were rushed through by Prime Ministers Tony Blair and Gordon Brown without a clear explanation of the purpose, it said.
The watchdog examined 51 reorganisations made during the period, putting the cost at £780m.
This represented an average expenditure of £15m per organisation.
Two of the new departments created since Mr Brown became prime minister in 2007 - the Department for Business, Enterprise and Regulatory Reform and the Department for Innovation, Universities and Skills - lasted less than two years.
The report contrasts the 25 new central government departments set up in the UK since 1980 - of which 13 have gone - with the creation of just two in the United States, both of which still exist.
The reasons given for Whitehall changes were generally expressed in "broad terms" without a clear explanation of the expected benefits, "creating the risk that some reorganisations may be unnecessary".
The report says: "The value for money of central government reorganisations cannot be demonstrated given the vague objectives of most such reorganisations, the lack of business cases, the failure to track costs and the absence of mechanisms to identify benefits and make sure they materialise.
"Overall, the value for money picture is unsatisfactory and the costs are far from negligible."
The head of the National Audit Office, Amyas Morse, said government workings were in a "constant state of change", bringing "inevitable" disruption.
He said: "We believe a more deliberate and carefully planned process makes sense before such costs are incurred and would also like to see a slow down in the rate of change."
Conservative MP Edward Leigh, the chairman of the Commons public accounts committee, which oversees the work of the NAO, said: "Designers of logos and makers of nameplates have had much reason to be grateful for central government's passion for constantly reorganising and renaming its departments.
"No one else seems to be very keen - especially the hard-pressed civil servants who have to cope with the fall-out while still trying to do their day jobs.
"Whether this frenzy of reorganisation and renaming gives value for money is entirely mysterious."
A Cabinet Office spokesman said: "Value for money must always be a priority, but it is also important that the prime minister is able to structure the government, acting quickly if necessary, to best develop and implement the government's policies and deliver on key priorities that bring real benefits to people and public services.
"Eighty-five per cent of the changes described in the report are not changes to Government departments, but to arm's length bodies.
"We have set out a clear programme of cuts and reforms to these bodies, making it more difficult to set them up and ensuring there are plans for them to be abolished when their purpose has been served."