Tourism Minister Selma Elloumi Rekik said the government would provide exceptional loans to help tourism businesses this year and next after Friday's shooting, which killed 38 people and prompted thousands of holidaymakers to flee.
"If tourism collapses... The economy falls apart," she told reporters.
On Friday, a Tunisian identified as 23-year-old Seifeddine Rezgui pulled a Kalashnikov assault rifle from inside a beach umbrella and opened fire on holidaymakers at a resort in Port El Kantaoui near Sousse before being shot dead.
Fears are now growing the attack, the second against tourists this year, could crippple Tunisia's tourist industry, which accounts for some seven percent of GDP and employs some 400,000 people.
"We can count, at least, with regards to the impact on Gross Domestic Product (GDP), on a loss of earnings of a billion dinars (USD 515 million, 460 million euros)," Elloumi said.
"I think that's just the minimum, but it's still an estimate."
France's travel agencies union said 80 percent of package holidays booked for Tunisia in July have been cancelled, with customers rushing to change destinations.
Visitors arriving in Tunisia on package tours from China, India, Iran and Jordan will now be able to obtain their visas on arrival at the airport, she added.
Tunisia's state budget for 2015 stands at some 29 billion dinars.
