See this post: Not possible to get reputation and cost required to get a WoW mount? - #4 by Schiller-1822
If you attempt to get the item info for item 161667, which is “Reins of the Voldunai Dunescraper,” you get a purchase_price with a value of 1.
This item actually costs 90000 gold.
This is the API call:
https://us.api.blizzard.com/data/wow/item/161667?namespace=static-us&locale=en_US&access_token=<YOUR TOKEN>
I have not tested other mount items, but it’s reasonable to assume other mounts have the same issue.