The Olympic Delivery Authority said the deal to build the pool complex would cost £242 million.
The centre itself includes the main swimming pool, diving pool and training pool.
And the firm will also build a 250 m bridge that will form part of the centre’s roof and connect the adjacent Stratford City development to the complex.
This carries a further price tag of £61 million and at its widest will be about 40 m across.
The original aquatics budget was estimated at £75 million and the £303 million represents a fourfold increase on that figure, which was originally contained in London’s bid document that it sent to the International Olympic Committee.
Balfour Beatty has formally signed the deal to build the venue - it had been sole bidder after rivals bidders Hochtief and Eiffel pulled out last year - and is expected to begin work in July.
The centre will have a capacity of 17,500 during the Games which will be cut to 2,500 after the event.
Nearly £500 million worth of funding for the media centre and the athletes village is being provided by the ODA, with these two schemes also being bankrolled by private money.
The media centre will be turned into industrial and commercial use in legacy mode while the village will form part of the wider Stratford City development, which includes a giant shopping centre.
The ODA said all its published budgets - it revealed the velopark would cost £80 million - were within its £6.1 billion budget, a figure excluding contingency costs.
This week ODA chairman John Armitt said the 2012 Games had to learn logistics lessons from the problems of Terminal 5.
Mr Armitt said: “It seemed to be the little things - like the time it took to get staff from the car parks - that combined together.
“But we have the advantage of nine months of preparing for the handover, rather than the week before.”
He said ODA observers at the Beijing Olympics would play a crucial role in monitoring both the run-up to the Games and how the event panned out. And he added: “The experience people bring back will tell us the extent to which our designs will be affected.”
He said one of the challenges facing the Olympics was that because of the opening ceremony, the first day was also the day of maximum usage.
Avoiding another Wembley
An independent dispute avoidance panel has been set up to help avert Wembley Stadium-style contractual disputes.
The expert panel has been formed to help the Olympic Delivery Authority and its contractors avoid potential disputes that could impact on the delivery of the Olympic Park and other London 2012 venues.
It will focus on finding solutions to problems that may arise before they become disputes requiring resolution.
Analysis: Time to dive in and start building
By David Rogers
For the ODA, the last bit of pre-construction price horror is out of the way. It still has to face the possibility that once construction starts budgets might be blown but that can be addressed later.
The £303 million figure is a lot for a swimming pool and some will inevitably compare it to the £75 million that was touted way back when the bid was made.
But it is worth remembering the ODA had nothing to do with this.
Chief executive David Higgins must be itching to get on with building the thing and it is noteworthy that the country’s best-known builders are all on the job in some way.
That is a good thing. A UK no-show would almost have been as bad as tales of budget woe.