▲你的句子中的 by 表示变化的幅度或程度，大家比较熟悉的相关用法有“增加了多少”和“减少了多少”（其中就用 by 来表示增加或减少的幅度或程度）。如：

The number of students increased by 1000. 学生的人数增加了1000。

Its output of cotton went up by 20 per cent. 其棉花产量增长了百分之二十。

The members decreased by 500. 会员减少了500人。

They cut down the expenditure by half. 他们把开支减少了一半。

The price was reduced by 18 per cent. 价格降低了百分之十八。

▲你的句子意思是：

Output may deviate from the average by as much as 30%. 产量与平均水平相比，其偏差可能会高达30%。（句中的 by 表示“偏差”的幅度或程度）

▲更多相似例句及用法（供参考）：

The speed can vary by as much as 15per cent. 其速度的差别可高达15%。

Temperatures can fluctuate by as much as 10 degrees.温差可达 10 度之多。

Under the new rules, some factories will cut emissions by as much as 90 percent. 按照新规定，一些工厂的减排量要高达90%。

Costs have been ratcheted down by as much as 50% since 1999. 自1999年以来，成本逐步降低了差不多有50%。

Output may deviate from the average by as much as 30%. 产量与平均水平相比偏差可高达30%。

Low consumer demand has forced us to mark down a wide range of goods, sometimes by as much as 30%. 低迷的消费需求迫使我们降低了许多商品的价格，有时降幅高达30%。