The BBC spent £102.5m last year on six TV soaps – including EastEnders, Casualty, Holby City and Doctors – a 20% drop overall in real terms in the past eight years.
The report on the cost management on BBC's continuing drama production by the National Audit Office for the BBC Trust also shows that EastEnders received an extra £500,000 on top of its £29.8m annual budget for last year's live episode to mark the show's 25th anniversary.
In the end the episode cost £696,000, with the difference being made up by economies on subsequent episodes of EastEnders.
The live 25th anniversary episode was broadcast on BBC1 in February last year and attracted 15.6 million viewers, the channel's highest audience since more than 20 million watched England lose to Portugal in the quarter-final of the Euro 2004 football tournament.
The NAO report shows that BBC Scotland charges its River City soap, made for Scottish viewers, the highest amount for overheads, representing 33.9% of the total production budget.
This compared with a 10.6% charge levied by BBC Vision on EastEnders, Casualty, Holby City and Doctors, and a 27% charge by BBC Wales for Pobol y Cwm.
The Scottish soap, which is also said to be a candidate for closure under the current 20% cuts being sought by the BBC director general, Mark Thompson, is also far more expensive than other continuing dramas examined by the NAO, costing £156,000 per hour, down from £210,000 two years ago.
This compares with £90,000 per hour for Pobol y Cwm, the Welsh language drama made by the BBC for S4C, and £109,000 for Doctors, the BBC1 daytime soap made in Birmingham.
The report shows that the efficiency savings of 3% a year plus inflation levied on Casualty, EastEnders and Holby City for the past three years, have led to reduced regular casts, from 18 to 17 in Holby and from 18 to 16 for Casualty.
However, it found that the average audience appreciation scores for the soaps rose, despite the economies, by 6.5%.
The NAO is concerned the BBC does not have a method of systematic cost benchmarking, to measure the comparative effectiveness of its soap and drama spending.
It is also concerned that a significant number of contracts are agreed with suppliers, without a second quote to ensure they are good value.
The NAO concluded: "We could not say whether the BBC is delivering value for money because it has not set audience-related performance objectives when deciding the future funding for individual continuing dramas."
It was also critical of the BBC's reliance on a metric called "cost per viewer hour" for its dramas, and points out that high production costs can be hidden by large audiences.
The NAO said this measure has significant drawbacks if the BBC is to adopt the "Delivering Quality First" strategy of fewer, bigger and better programmes. It may be that higher spending results in programmes more suited to frequent repeats.
The analysis shows that EastEnders costs 3.5 pence per viewer hour, while Casualty was 9.4 pence per viewer hour. Casualty also spent the highest amount, 17% of its budget, on stunts and special effects.
The BBC has rejected the suggestion of audience-related performance objectives describing them as micro management and excessive bureaucracy.
Amyas Morse, head of the NAO, said: "The BBC is doing a good job of applying basic financial controls and achieving steady cost reduction across its portfolio of continuing dramas. However, it should take a more holistic approach and compare the cost of these programmes with audience levels and opinions, both targeted and achieved, as these are critical measures of broadcast performance, and a key guide to whether the continuing dramas are delivering value for money."
Anthony Fry, BBC trustee with lead responsibility for value for money, said: "Popular, long-running dramas like EastEnders sit at the heart of the BBC schedule, generating loyalty from audiences as well as an environment for new writing, acting and production talent to flourish. They cost viewers less than they did 10 years ago, while audience approval is on the up.
"We accept the majority of the NAO's recommendations, with the exception of the suggestion that the BBC should set targets at drama level. We believe there is a risk that this could harm the BBC's ability to produce distinctive programming by acting as a disincentive to take creative risks and creating unnecessary bureaucracy."
• To contact the MediaGuardian news desk email editor@mediatheguardian.com or phone 020 3353 3857. For all other inquiries please call the main Guardian switchboard on 020 3353 2000. If you are writing a comment for publication, please mark clearly "for publication".
• To get the latest media news to your desktop or mobile, follow MediaGuardian on Twitter and Facebook.
