One of the primary questions my American friends ask me is how expensive it is to travel from country-to-country within Europe. When I tell them it's cheap — often under $50 for a one-way ticket — they're shocked. Of course, it helps if you have some flexibility when it comes to your travel dates.
My favorite go-to flight website is Skyscanner, where you can check out a whole month's worth of flight dates at a time — or even several months' worth.
One moment, a last-minute flight from Madrid to Berlin, for example, may be around $200, but the next, later that day or the following day, it may be around $50. And then it may increase again.
Since the prices tend to fluctuate, if you don't want to keep checking the site yourself, you can set up flight alerts that'll let you know when a fare increases or decreases in cost.
It also pays to check out European-based budget airlines, like Ryanair — which often has flights for $10 one way — and Wizz Air. They, too, can be lifesavers.