I don't know why it's so cheap, but whatever
Check this
Plus, if you really want this game - add me on Steam

你好，这是来自一个中国的卖key网站。我不确定你能不能注册。但我看了上面那位名叫ZOMBIE0q的朋友发的链接，发现他发的价格要更低，所以我推荐你点击他的链接购买。
DeepL translator :
Hi, this is from a Chinese site that sells keys. I'm not sure if you can register. But I looked at the link posted by the friend named ZOMBIE0q above and realized that the price he posted is much lower, so I recommend you to click his link to buy it.