Define the difference between low and high, and relate it to the US.
250 persons per square mile.
Right now the average population density for the entire US is about 84 ppsm; although there are currently 11 states at or above 250 and 10 more getting close. Not to mention that many urban areas are way above this, in the tens of thousands of people per square mile.
Since people tend to live in cities, the effective population density is much higher than just the total population divided by the total land area of the U.S. For instance, Kansas City has a population density of about 1500 ppsm.