Airfares are often the barrier that keep would-be visitors from the sidewalks, byways and plazas of Europe. Many people get around that by using frequent flier miles, or by employing other techniques such as scanning airline deal pages. During the winter months, it's sometimes possible to find round trip flights between North America and Europe for as little as $600. If you can't go during that time of year, the shoulder seasons of mid-Spring and mid-Fall often provide relief from the expensive fares found in Summer.
Another tip: sometimes, flying through Ireland or Iceland can provide a better fare. Those places make great stopovers, too. Many who have made a brief visit to either locale find themselves wanting to go back for a longer stay.