William Larson, an economist with the Bureau of Economic Analysis, says the answer is $22.98 trillions.
That amount doesn't include Alaska or Hawaii and only measures the 48 contiguous states and the District of Columbia (around 1.89 billion acres of land). The $22.98 trillion amount is also only for the value of the land, nothing else. It doesn't account for the value of buildings or bodies of water.
How did William Larson arrive at the amount?
If you want the actual paper and calculation behind the figure, it's available at the Bureau of Economic Analysis at http://www.bea.gov/papers/pdf/new-estimates-of-value-of-land-of-the-united-states-larson.pdf. Note that his estimates reflect the land’s value back in 2009 (during the recession) so the land should technically be valued a lot more now. Mr. Larson believes that his estimates carry a margin of error of 10%.
Personally, it seems kind of low to me. I thought that it'll be pass the trillions range and in the quadrillion or quintillion range. Kind of makes you wonder how much other countries are worth.