Amro said the figure included only "direct damages" to the Gaza economy and warned it could climb further once additional impacts on the 1.8 million population are taken into account.
A more precise assessment would be carried out once calm returns permanently to the overpopulated sliver of territory where more than 1,850 people were killed and nearly half a million displaced, he said.
Amro said international donors were expected to meet in Norway in September, but gave no further details.
The fighting began on July 8 and expanded into an Israeli ground offensive on July 17.
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Hundreds of homes were destroyed and last week the Gaza Strip's only power plant was shelled, imposing total blackouts.
The Palestinian territory, under blockade by Israel and Egypt, suffers from chronic water and power shortages.
Even before the latest fighting, Gaza suffered power cuts of eight to 12 hours a day, hampering hospitals, schools, businesses and water treatment centres.