Once you're over there, airfare is not necessarily super-expensive.
And sometimes, depending on where you are, the train is just fine, too.
But only you yourself can figure out an estimate.
Look at an atlas and/or guidebook, and decide which place interests you most.
Then do some more research.
US Dept of State, Travel Section
Eurotrip
The sites below could be very helpful in terms of cheap transport.
In 2006, we traveled to Germany, France and Croatia in a little less than 3 weeks, by train and air, and the airfare was not that bad.
Which Budget Airline
EuropeByAir
For lodging, you can use either
TripAdvisor or
Eurocheapo.