Gordon Brown has been told by First Minister Ian Paisley successive British governments were to blame for leaving Northern Ireland's economy in a "mess".
Mr Brown was in Northern Ireland for the first time since becoming prime minister, attending a British-Irish Council summit at Stormont.
Mr Paisley said they wanted more talks in the coming months with Mr Brown over an enhanced financial package.
Mr Brown earlier said the £51.5bn funding was "very big indeed".
"It is to make possible the current expenditures of the Northern Ireland Assembly and to make possible a big programme of investment in the future," he said.
Speaking after the meeting, Mr Paisley said the executive would continue to work to get the right package.
Mr Paisley blamed Mr Brown's predecessors in government for misspending EU funding.
"The south of Ireland used the monies they got very well indeed but we did not get the money, it was used to pay the ordinary government expenses of Northern Ireland," he said.
'Work in progress'
Mr McGuinness said their economic efforts were "a work in progress".
"The prime minister was at pains to say that he is here to help and that he recognises the economy is of critical importance," he added.
Taoiseach Bertie Ahern was also at the summit, as were Scottish First Minister Alex Salmond and Welsh Deputy First Minister Ieuan Wyn Jones.
Mr Ahern said the summit was "another important day in the working of the institutions of the Good Friday Agreement".
The British-Irish Council was set up under the Agreement to focus on matters of common interest such as the environment, social inclusion, tourism and transport.