Challenge Coins surfaced during the World War I era.  The practice of carrying a coin designed specifically for a unit was popular with the Army Special Forces.  Carrying the coin at all times and presenting it when "challenged" to prove affiliation with that unit resulted in a number of consequences for those who could not produce a coin; the most popular required the coinless soldier to buy a round of drinks. That practice continues to be popular today. 
Prices might vary on certain coins.