Asked if the deal would reduce the prospects of a powersharing agreement being secured in Northern Ireland, Mrs Foster replied: "Not at all".
The Conservatives have 317 seats in the 650-seat parliament after the June 8 election and need the support of the DUP's 10 MPs to be able to govern.
Ian Blackford, leader of the Scottish National Party's 35 members of parliament, said Scotland should also get its "fair share" of funding and accused May of discovering a "magic money tree" after years of austerity cuts.
The deal is a "confidence and supply agreement", meaning that the DUP will only guarantee to support the Conservatives in confidence and budget votes.
Every time the government says there isn't money available for something, critics will point out that they found money for Northern Ireland to get the DUP on board.
Earlier this month, the United Kingdom held a snap election that May chose to hold last March as she was riding high after the citizens voted to leave the EU.
The accord was sealed on Monday morning in London after talks between May and DUP leader Arlene Foster at Downing Street.
However, Scotland's First Minister Nicola Sturgeon tweeted: "Any sense of fairness sacrificed on the altar of grubby DUP deal to let PM cling to power, & Scots Tories influence in No10 shown to be zero".
"So the agreement we have come to is a very, very good one, and look forward to working with you".
Opposition Labour Party leader Jeremy Corbyn said the agreement suits May's wish to stay in power but does little for the country.
May has been in talks with the Democratic Unionist Party to try to shore up her government after a disastrous election in which she lost her majority in the House of Commons.
May and Foster looked on Monday as the agreement was signed by the Conservative Party's chief whip Gavin Williamson and the DUP's Jeffrey Donaldson.
Foster said the funding would "address the unique circumstances" of Northern Ireland.
This Tory-DUP deal has not been done in the national interest but in the interest of @Theresa_May & @Conservatives' own political survival.
Northern Ireland has been in crisis since Sinn Fein pulled out of its government in January, prompting an election in March and a series of missed deadlines to restore the compulsory coalition between Catholic nationalists and pro-British Protestant unionists.
As part of the deal, the government will provide funds to boost Northern Ireland's economy, while investing in infrastructure, health and education.
Labour branded the deal "shabby and reckless", and warned it would undermine the trust in the impartiality of the British Government which was vital to the implementation of the Good Friday Agreement.
"Austerity has failed. Cuts to vital public services must be halted right across the United Kingdom, not just in Northern Ireland".