According to this reference, grassfed beef can be stocked at 1.6 steers per improved pasture land acre and go from 525 pounds to 1050 pounds from May to October. That means about 0.625 acres of improved pasture per finished steer. Unimproved pasture takes more, seemingly 2 – 4 according to forums I found.
According to this description of pounds of corn per pound of beef, they estimate that it takes 17 – 62.5 bushels of corn to finish a steer. Let’s say we can produce 140 bushels of corn per acre. That means that it takes 0.13 – 0.45 acres to finish feedlot beef. I also learned that 6 pounds of feedlot feed contains 1 pound of corn and the balance includes distillers grain left over from making ethanol and corn gluten from high fructose corn syrup. So the corn nutrients are extracted in a variety of different methods before they reach us. Cows are brought to the feedlot at 600-900 pounds and are slaughtered at 1300 pounds, which means that they are generally foraged for a while first and slaughtered larger than grass fed beef.
It seems like it probably takes about three times as much land to have a grass fed steak vs. a feedlot steak.