The points value of a cavalry model depends on the rider's worth as well as the inherent fighting abilities of the steed. Due to his extra mobility a cavalry model is deemed to be worth double the points of the same model on foot, including the value of his armour and weapons. A man on a horse would cost 10 points against 5 for a man on foot, for example. If the man had a shield his value would be 6 on foot and 12 riding a horse.
Most steeds are worth extra points compared to a horse because they have their own attacks. A Warhorse is therefore worth an extra +3 points, a Giant Spider +4 points, and so on. These values are included in the list above. So, a man riding a Warhorse costs not 10 points but 10+3 = 13 points. This value does not include the cost of armour or weapons of course.