Corruption costs Greece about 8% to 10% of GDP per year.

A Brookings Institute study by Daniel Kaufmann estimated that corruption cost Greece about 8% of gross domestic product per year. (Some have estimated it as high as 10%.) Notably, a large bulk of it involves ordinary citizens doing day-to-day activities.
"If Greece had better control of corruption — not to Swedish standards, but even at Spain's level — it would have had a smaller budget deficit by 4% of gross domestic product" on average over the past five years, Kaufmann wrote back in 2010.
A 2014 study by the European Commission said Greece was the most corrupt country in the European Union — on par with China.

Source: Greece's Horizons: Reflecting on the Country's Assets and Capabilities