你的翻译有三个问题：第一,“外商”漏掉了,第二, could reduce to改为could be reduced to (要用被动式), 第三,20 million less than (that of the)eastern region,应添加括号内的内容.
修改如下：
The requirement for the registered capital of a foreign business investment project in the western region could be reduced to 30 million RMB, 20 million less than that of the estern region.
20 million less than eastern region这种口语式说法也是可以接受的,但尽量避免在正式的文字中出现,此句作为补足语（对比前面的数字）,省略了that of the,另外还省略了主语--外商投资资本数额要求.
供参考.