Is there a legally mandated filing fee and/or deposit?

Article22 of the law 345/10,06,1976, Official Gazette vol. A issue 141

A fee is required in order to submit a complaint document before theSpecial Supreme Court. This fee shall be equal to the fee required by theSupreme Court for the same type of document.

http://www.ministryofjustice.gr/site/kodikes/%CE%95%CF%85%CF%81%CE%B5%CF%84%CE%AE%CF%81%CE%B9%CE%BF/%CE%9A%CE%A9%CE%94%CE%99%CE%9A%CE%91%CE%A3%CE%A4%CE%9F%CE%A5%CE%91%CE%9D%CE%A9%CE%A4%CE%91%CE%A4%CE%9F%CE%A5%CE%95%CE%99%CE%94%CE%99%CE%9A%CE%9F%CE%A5%CE%94%CE%99%CE%9A%CE%91%CE%A3%CE%A4%CE%97%CE%A1%CE%99%CE%9F%CE%A5/tabid/356/language/el-GR/Default.aspx

http://www.hellenicparliament.gr/UserFiles/f3c70a23-7696-49db-9148-f24dce6a27c8/PD%2026-2012.pdf