Source: Warhammer Fantasy: 4th Edition

Heroes and Mounts (Points Value)
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Heroes and wizards can ride a horse, warhorse, or other appropriate steed. The cost of the model is always the value of the hero or wizard plus the extra cost indicated for the mount. The value of the rider is not doubled as this would make mounted characters far too expensive. This means that a character can ride a horse for free, or a Warhorse for an extra +3 points. This may seem a little cheap but it is only right and proper that heroes get to ride where others might be obliged to go on foot.

To find the cost of a heroic character or wizard riding a monster simply add the two values together.