The phrase "astounding sum" is correct and usable in written English.
It can be used to describe a large or impressive amount of money or value, often in contexts related to finance, donations, or costs.
Example: "The charity event raised an astounding sum, exceeding all expectations and setting a new record for donations."
Alternatives: "incredible amount" or "remarkable total".
In Calcutta, she raised the astounding sum of fourteen thousand pounds for the relief of the Irish famine.
Going 81-81 would require them to win 38 more games than last season, an astounding sum.
Following the 1988-89 season, the Hawks signed Koncak to a six-year, $13 million dollar deal, an astounding sum at the time.
The Italians and Welsers got some of the business, but Fugger financed five hundred and forty-four thofsand of it — an astounding sum at the time, enough to fight a couple of wars.
At the turn of the 20th century, in response to this epidemic in the US, John D. Rockefeller established the Rockefeller Sanitary Commission for the Eradication of Hookworm with a million dollar grant – an astounding sum at the time.
Go on to the astounding sum he is said to have promised to raise for "Rebecca": $4.5 million, easily 10 times more than the wealthiest regulars normally put down for a show.
The corporate and financial elites threw astounding sums of money into campaign contributions and high-priced lobbyists and think tanks and media buys and anything else they could think of.
At its peak, toward the end of last year to January, ICOs were garnering frankly astounding sums of money as investors sniffed around for potentially massive return on investment.
There is certainly an argument to be made that the lending/austerity path should never have been taken in the first place at the outset of the Great Recession, given how unrealistic it always was that the astounding sums of money lent were never going to be repaid.
That sum is astounding, and one of the largest showings ever, but it doesn't even tell the whole story.
