A room in Athens or on an island ranges from around €80 to over €500 per night depending on the time of year, location, type of accommodation, and amenities. Santorini is known for being an expensive island.
For romantic honeymoons, the village of Naoussa on Paros is very popular and less crowded than Santorini. There are plenty of agencies specializing in honeymoon packages in Paros and Santorini. But also be careful with your spending in Naoussa, which is now a village where you'll find high-end stores selling luxury brands.
A meal in Greece ranges from approximately €10 for a souvlaki with a beer to more than €30 / person for a full meal with a bottle of good wine and fresh fish. A cocktail (for lovers) ranges from €8 to €15.
You can also book accommodation with a kitchen, shop in supermarkets, and prepare your own meals/breakfasts, which will significantly reduce your food expenses.
An economy flight from Athens to Santorini or Paros ranges from €50 to over €140 depending on the time of year, with or without checked baggage.
An economy ferry ticket costs from €40 to €100 depending on the route and ferry type.
Car rental on the islands ranges from €30 to €80 per day depending on the type of car and the time of year.
A public bus ticket on an island usually costs less than €2.50. Two taxi rides on Santorini can be as expensive as a day's car rental.
Do the math.