The temporal phrase in the past two days defines the time-span from the point-of-view of the present moment. That span is "attached" to the present moment, like a cart to a horse or a tail to a kite. It refers to the time including and immediately preceding the present moment.
For that reason, we use the present perfect, has gone down.
By contrast, the price of butter went down two days ago.
P.S. The deciding factor is whether the time-phrase is connected to the present; the connection to the present is the key factor:
The height of the mountain range has changed over the last 400 million
"Over the last 400 million years" includes today and includes the span of past time of 400 million years reaching up to today.
If the sentence lacks a specific time reference, the tense establishes one that includes the present.
The height of the mountain range has changed over time.
In the sentence above, the speaker is referring to a mountain range that exists today, whose height has not been constant. In the sentence below, the speaker is referring to the mountain range as one which has stopped changing, or as one which no longer exists, which can indeed happen over the course of geological time:
The height of the mountain range changed over time.