In the movie War Horse, a horse is purchased at auction for the price of 30 guineas. In the time the movie is set, circa World War I, 30 guineas were equal to £31 and 10 shillings. In 2012 this would equal £1,627.67 or $2,153.68.
Before 1971, British money was counted on a different scale than it is today. It was based on:
1 pound = 20 shillings;
1 shilling = 12 pennies.
A guinea was worth 1 pound 1 shilling. At auctions, the winning bidder pays in guineas but the seller is paid in pounds. The auction house keeps the extra 1 shilling per pound as commission.