The high cost of airfare is why repositioning transatlantic crossings are so popular. Some of the prices on these are incredibly cheap.
But on more than a few occasions, we've found roundtrip airfare to be cheaper than one-way airfare. Plus, when considering what airport to fly into or out of in Europe, look at several options. Frankfurt is the largest, but you should also look at Paris, Munich, Amsterdam, etc. And while sometimes you can get good airfare flying into Heathrow, flying out is more expensive because of the taxes.
But with train fare being fairly reasonable in Europe and the major airports not too far away from each other, there are often more choices than people think about doing.
And if going out a smaller city, definitely think about trains. For example, we were leaving out of Venice and after researching all the options, it was much cheaper to fly into Munich and take the train than it was to fly into Venice.