This is pure speculation, but there is evidence that their price is wrong.
Bison and aurochs were priced on the Animal Archive Player Companion, along with many other domesticable animals, like several on this list on the SRD.
When reared from birth, the following animals can easily be ridden by Small or Medium humanoids, depending on the riding animal’s size. In addition, most of these animals can be purchased already trained for combat.
However, considering the description of both animals, I am inclined to believe their prices were swapped by accident during the writing of the book.
On aurochs, it says:
Aurochs (both singular and plural) are large herd animals similar to domesticated cattle.
While on bison, it says:
Bison are large herd animals. They can grow as long as 12 feet and as tall as 7 feet at the shoulder. Average bison weight 2,200 pounds. The bison’s statistics can be used for almost any larger herd animal, such as the buffalo.
By reading that, we can say the aurochs stats can be used for any common herd animals, like cows, bulls and other similar animals. While bison stats can be used by heavier herd animals, like buffalo and bison, as they are really similar animals.
As for aurochs, the core rulebook already gave us a price on domesticated cattle, 50 gp, with the following description:
Price 50 gp; Weight 1,500 lbs.
Cattle are kept for milk and meat and used as beasts of burden. A bull is used primarily for mating. Other males are castrated and become oxen, which are used to carry loads and plow fields.
This price is the same as what was published for bison on the Animal Archive (50 gp), while aurochs were priced as 300 gp (6 times more expensive). And I honestly see absolutely no reason for them being so much more expensive when they have a lower value as cattle (1500 lbs vs 2200 lbs) and are probably harder to domesticate being more aggressive than aurochs and cattle. Even a leopard costs only 100 gp.
People being able to purchase animals has even caused a problem on Pathfinder Society a few years ago, especially when a first level character could start the game with a CR 4 combat-trained buffalo/bison (exactly the topic here), or a 100 gp leopard that could easily learn the attack trick, which caused a new rule to be added on the Additional Resources specifically about the Animal Archive, saying:
Pathfinder Player Companion: Animal Archive
[...] Finally, a PC can only purchase an animal, mount, or similar creature if its Challenge Rating is lower than that character's level; creatures with a Challenge Rating of 1 or lower are exempt from this restriction, as are horses. [...]
If we compare the CR of several of those animals with their price, we will notice that they are priced relative to their CR, except for aurochs and bison:
Aurochs, 300 gp, 450 gp for combat trained. CR 2
Bison, 50 gp, 75 gp for combat trained. CR 4
Tiger, 325 gp, 500 gp combat trained. CR 4
Lion, 200 gp, 300 gp combat trained. CR 3
Leopard, 100 gp, 150 gp combat trained, CR 2
Boar, 100 gp, 150 gp combat trained, CR 2
Elk, 100 gp, 150 gp combat trained. CR 1
But considering the rules-as-written, this is perfectly legal and the price is official. Player Companions rarely get an errata, as they hardly, if ever, are reprinted. This has happened only a few times, noticeably with the first Adventurer's Armory and the Technology Guide.
However, with Ultimate Wilderness around the corner, it was previewed on 2017 PaizoCon that rules and stats published on both Animal Archive and Familiar Folio will be reprinted, and we could see an errata about this.