How much would an average holiday in greece cost for 2?

I am 16 1/2 years old, The friend I am going with is 17 in a month or two. I met this girl on the internet who lives in greece, and our primary reason for going is so I can meet her. However I have not had a holiday in a long time and was curious to how expensive everything would be. That is two return flights to athens (where she lives), accommodation, food, + spending money. We may be going around july time, failing that - after christmas. We plan to go for either a week or two weeks, so Information on both would be helpful. Thank you for your time :). If there is anything else you need to know then ask in your answer and I will provide the information.