Mishna: The mitzva of the two Yom Kippur goats, the goat sacrificed to God and the goat sent to Azazel that are brought as a pair, is as follows, ab initio: That they will both be identical in appearance, i.e., color, and in height, and in monetary value, and their acquisition must be as one, i.e., they must be purchased together. And even if they are not identical, nevertheless, they are valid. And similarly, if he acquired one today and one tomorrow, they are valid.
The Tosafot Yeshanim point to a Gemara in Sanhedrin, which says that no two individuals are truly identical, and ask how two identical goats can possibly be found. The answer they suggest is that we must distinguish between people who have clearly identifiable characteristics and animals whose appearance may be much more similar. Nevertheless, they refer to a comment in the Jerusalem Talmud that seems to indicate that no two things will ever be identical – even two grains of wheat have differences between them. This leads the Tosafot Yeshanim to conclude that the Mishna merely means that the two animals should be as similar as possible in their general appearance.
With regard to value, according to the Jerusalem Talmud, we are not concerned about their actual selling price, but rather about their true value. Even if they were purchased for different amounts of money, as long as they are of equal value we have met the requirement; if their values were significantly different, even if they cost the same amount of money (e.g. one of the sellers gave a discount to the Temple representative) the requirement would not be met.
Finally, the Mishna recommends that they be purchased at the same time, and the commentaries explain that ideally they should even be purchased from the same merchant, as we try to limit anything that distinguishes the two animals from one another.