According to this article, in 2009, around 13 million people from the United States travelled overseas, of which 35% visited Europe. Given the United States' population of 320 million, we can estimate that currently, around 1.4% of Americans visit Europe annually.
What would that percentage have been in the mid- to late 19th century? All transatlantic travel back then was by ship, and likely considerably more expensive, so presumably, the fraction of Americans able to afford a trip to the Old World was much smaller than it is today. However, out of those who could afford to travel overseas today, only a relatively small fraction actually do, so that does not necessarily mean much.
Note: In the period that I am asking about, millions of people immigrated to the U.S. from Europe, and had thus "visited" Europe before they every reached American soil. Those people should not count towards the total unless they returned to Europe afterwards, with the intent to return to America later. A visit is a trip with the intention of returning.