Demand must be greater than supply.
The edition size of the Nadelman “Horse” was 6.
Because more than one person who in the year 2000 could spend in excess of $200,000 for this sculpture, it sold at auction for more than $300,000.
Elie Nadelman was “a well-recognized sculptor” meaning his sculpture is included in textbooks chronicling 20th century American sculpture. Additionally, his sculpture is distinctive and its appearance presented a look that was later readily identified with the term “modern sculpture”. Nadelman’s contribution to the field of sculpture was relatively unique. This uniqueness contributed to Nadelman having earned a spot in American art history of the 20th century.
Another important American sculptor of the 20th century was Alexander Calder. Calder pioneered sculpture formats known as stabiles and mobiles, often using primary colors to complement black elements. Calder produced highly distinctive work, pushing the boundaries of sculpture produced prior to his participation. 
Corinne Cain of www.SavvyCollector.com