Why does the grandma jump up and shout "杀了俺俺也不信！"?
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