The table below indicates square footage and sales price for multiple houses in the United States. If I build a 500 square foot addition, how much

The table below indicates square footage and sales price for multiple houses in the United States.

If I build a 500 square foot addition, how much would my home value increase? 

What percentage of variation in home value is explained by the variation n house size? 

A 3000 square foot house is listed for $500,000, is this price normal for real estate values in the US? 

What is the cause of this discrepancy?