As of 2014, the minimum cost of building a mansion is around $1 million. This cost can vary greatly depending on the cost of the lot on which the home is built, the total square footage of the home, the design of the home and the types of materials used.
The average cost of building a house is between $80 and $120 per square foot. Assuming anyone building a mansion desires top quality, the cost of construction is probably closer to $120 per square foot. Although the term "mansion" technically describes a house's grandeur rather than size, American real estate professionals classify any home with more than 8,000 square feet as a mansion.