Why does the grandma jump up and shout "杀了俺俺也不信！"? 我小的时候，有一段很特别的时期。有一天，我父亲对我姥姥说，一亩地里能打三十万斤粮食，而我的姥姥，一位农村来的老实老太太，跳着小脚叫了起来：“杀了俺俺也不信！”她还算了一本细账，说一亩地上堆三十万斤粮，大概平地有两尺厚的一层。当时我们家里的人都攻击我姥姥觉悟太低，不明事理。我当时只有六岁，但也得出了自己的结论：我姥姥是错误的。……
王小波，积极的结论，一只特立独行的猪

Here is my translation of the part in bold:

杀了俺俺也不信!
My translation: [Even if] you kill me, I won't believe [you]!
Google Translate (v1):  I didn't believe it if I killed it!
Google Translate (v2): I don’t believe you killed me!

This seems like an peculiar reaction to the price of grain, particularly from an old lady (even if she is familiar with the price of grain, and is a bit crazy).  I feel like there is some significance here I'm completely missing.
Question: Why does the grandma jump up and shout &quot;杀了俺俺也不信！&quot;?
there is some significance here I'm completely missing

well, you mis-interpreted the charatcter “打”  to price.

一亩地里能打三十万斤粮食

“打” is “打榖”, it means harvest
the quoted verse was a common “fake news” in china, during great leap forward (~1958-1962), roughly,
[in] one 畝 (about 667 sq meter) field, one can harvest 300,000 斤 (500 grams x 300000 —&gt;  150,000 kg) grain.
the sad fact was, most people believed it.
btw, the highest record of such fake news is 130435 斤
https://zh.wikipedia.org/zh-hk/放卫星