Detail
LFCarry: 'Blizzard has not published the gold cost at 40 or at 60, or whether those remain the unlock levels.' Classic 1.12 baseline: level 40 apprentice riding for 60% speed (about 100g total with mount), level 60 epic riding for 100% speed (about 1000g total). Pack mounts such as the Cerulean Prideclaw still need the riding skill.
Confidence note
Classic-baseline for the numbers; Forever unpublished