What's a reasonable average cost per day in Greece for lodging and food?
Hotels range from 25-1600 euro/night. What class of hotel and what are the amenities you require? What sort of food? When will you be there? The range is just too wide to estimate without any information.
You can do a Google Search for hotel prices and can see what they have to offer from their websites.
This prior topic on Rick's estimates of trip costs isn't specific to Greece but may be helpful:http://www.ricksteves.com/graffiti/helpline/index.cfm/rurl/topic/9097/your-europe-trip-budget--not-just-for-first-timers.html
Adding to Lee's point it would also be useful to know how many are in the party.
That obviously effects hotel prices but, in my experience, also effects the price of meals. My partner and I pay 30 - 35 euros for an evening meal when we are in Greece together but when I'm on my own I end up paying 20 - 25. The difference is that we always share side dishes such as salads when both of us are there, and that I still order one when I'm on my own even if I don't eat it all.
It will vary wildly by location as well, though even on Mykonos and Santorini you can find cheap places to sleep, just don't expect a Caldera view or beachfront. Still, compared to the rest of Europe, Greece can be done economically. It seems I have better places to stay and much more to eat for my Euro. I rarely recall a meal for two over 30 Euro and I was usually able to stay around the 50 euro mark for a room, often less. Now that has been a couple of years, but with the economy I don't think it has exploded in cost.