Campaign group Republic has published its fully revised report on royal finances, "Worth Every Penny?", detailing the hidden costs that include Duchy income, costs met by local councils and the sovereign support grant.
The total comes to £299.4m for 2013/14, around 9 times higher than the official figure due to be published on Wednesday.
The report can be downloaded at www.republic.org.uk/wortheverypenny.pdf.
Included in the hidden costs are Lord Lieutenants which set the taxpayer back more than £2m a year - despite most people having never heard of them. The report also includes lost profits from the Duchy of Cornwall, an estate that should be sending its revenue to the taxpayer rather than Prince Charles.
Local authorities are estimated to have spent over £21m last year on royal visits.
Republic's chief executive officer, Graham Smith, said today:
"These figures point to a royal household out of control and beyond proper scrutiny. This is what happens when public figures can't be challenged and when they are protected by official secrecy."
"Far from being good value the British monarchy is one of the most expensive institutions of its kind in Europe. Despite dubious claims about tourism the monarchy is all cost and no gain."
"When public services, flood defences and jobs are being cut, this kind of spending on the royals is a scandal."
"A key point here is that the total cost is hidden - it shouldn't be down to us to work this out, all these costs should be properly accounted for and reported independently of Buckingham palace. This week we'll see another official report that ignores tens of millions of pounds and which is couched in spin and fantasy."
"MPs need to urgently investigate these hidden costs and look at radically changing the way the royals are funded. These figures shouldn't be a surprise when we see millions wasted on travel and refurbished palaces. Drastic action is needed to stop this abuse of public money."
"All we need is an annual salary for the head of state and a modest budget for managing official duties. The British taxpayer does not owe the Windsor family a living."
The report will say that the annual cost of the monarchy is equivalent to 14,000 new nurses or 13,000 police officers. The cost is more than the amount cut from flood defence spending over recent years.
Claims about tourism and Crown Estate revenue are also taken to task in the report. There is no evidence that the monarchy generates any revenue for the country - even the bogus £500m tourism figure often quoted in the press represents 0.03% of GDP, less than the margin of error of 0.7%