A phrase in baseball that refers to a run scored as the result of a technicality, a bad or otherwise sloppy play, or another means of scoring not directly related to solidly played baseball. An example of a cheap run is when a pitcher walks four batters in a row, thereby forcing a run home. Since the offensive team did relatively little to earn the run, it is considered "cheap". Players and fans have different opinions of cheap runs. The players, while they would like to avoid causing them, rarely mind scoring them. Fans, on the other hand, prefer to see well played baseball with as few cheap runs as possible.
The phrase "cheap run" comes from the understanding that if something is "cheap" it is probably not "worth it", whatever "it" happens to be. Scoring opportunities are usually earned through good baseball strategy and superior play, so when a run is scored which was not the result of that, it is considered cheap because it wasn't earned. However, a cheap run is not the same as an "un-earned" (versus "earned") run, which is used in the calculation of a pitcher's ERA. In this case, "earned" is a matter of perspective.