The phrase 'formidable sum' is correct and usable in written English.
You might use it to describe a large amount of money, especially when it is an amount that is difficult to come up with. For example, "Paul needed to come up with a formidable sum of $20,000 to pay his medical bills."
Mr. Juck lost 25 of Montenapo's original 60 employees, and a formidable sum of money he declined to reveal.
He has pledged to spend millions by the primary election in September — not at Mr. Bloomberg's level, but still a formidable sum against opponents who, lacking personal fortunes, have agreed to abide by spending limits in exchange for public matching funds.
Still, this week Zopp's campaign leaked that it had already raised $665,000, a formidable sum.
Other countries including China, India, Israel, France and Pakistan are expected to devote formidable sums on tactical and strategic missile systems.
They also wonder if any change can provide enough resources to enable candidates to compete with those who can raise formidable sums without federal help.
The impression remained throughout his life, even among many of his supporters on the left, that he was somehow less than the sum of his formidable parts.
The complex of policies that pushed the Philippines into the economic quagmire over the last 30 years can be summed up by a formidable term: structural adjustment.
Nothing better sums up the low-key but formidable political power of Hensley & Company than its response to two recent ballot initiatives.
By any standard, the sums being raised this early in the campaign are formidable.
As far as summing up the conference, this is of course a formidable task, because so much ground has been covered.
Those very simple, bleakly articulate words sum up the hopes and hopelessness that infuse this shy and formidable little play.
