Many restaurants add a service fee – check on their menu, it should be displayed. Tips are not obligatory but wages in Greece are low compared to many European countries and many locals only gain employment during the tourist season, so tipping is always welcomed, especially if you have received good service. Leave the tip on the table so that it is clear it is for the staff.
During the 2018 season you could expect to spend on average approx. €18-22 p.p. for a starter, main and drinks. Of course if you choose fresh fish, or steak the prices will vary! Local wine is a good option and is much cheaper than bottled wine.