Costs
The strength of the Euro will make perhaps the biggest difference to foreign visitors, but even then, Athens is, by regional and global standards, fairly expensive.
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At the budget end (staying in hostels or Couch Surfing, rarely eating out, walking and visiting mostly free sights), you might be able to get away with about €30-€40 a day.
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A more average budget for most visitors (staying at standard hotels, AIRBNB or family run guest houses, eating out once a day, going to the full range of sights and taking public transport), would be around double that, at €60-€80 a day.
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Double that again and over €200 buys a pretty high-end experience with smart hotels, meals at the full range of restaurants and taking taxis around town whenever you feel like it.
Note that traveling solo will probably raise any of these per-person-costs by at least a third for all except budget travellers.