This is a frequently asked question and a good one. But we're not able to accurately predict your trip cost, because so much depends on your choices you make.
But that doesn't mean we can't help some. In his book Europe Through the Back Door 2009, Rick Steves gives a rough rule of thumb of $185 per day per person for total trips costs including everything except airfare.
The $185 per person per day includes these assumptions:
a couple sharing a $150/night hotel room (thus $75/person)
breakfast included with the hotel rate
lunch $15/person
dinner $25/person
$5 misc snacks/person
That's $120 per person per day for hotel and food.
The $185 per person per day does not include airfare and assumes a couple staying in middle of the road hotels, eating one sit down meal per day, and traveling generally middle of the road.
Some forum members are (understandably) proud of the fact that they do trips for less than Rick's guidelines (we don't seem to hear much from those admitting they spend more than Rick's $185/day). But you may want to be cautious about evaluating implications that because others, who travel in a particular way and carefully control costs, can do it for less, you can too--of course it can be done for less (see below, Rick addresses that)--but in travel it's probably true that you don't consistently get what you don't pay for.
Doing it for less means choices have to be made. Rick's book gives an alternative budget, for what he calls "student or rock bottom budget travelers," of about $100 to $110 per person per day total cost, again this includes everything except airfare. This is based on a single traveler staying in a hostel or equivalent dorm type of sleeping accommodations and being very careful about the food budget and not eating regularly in sit down restaurants.
