1. Expression ：hit pay dirt
Explanation 1：To get great riches. （发大财）
Example：After years of poverty, the writer hit pay dirt with his third novel.
Jane's doing well. She really hit pay dirt with her new business.
John has hit pay dirt with his carpet cleaning business.
Explanation 2：To discover something of value. （发现有价值的东西）
Example： I tried to borrow money from a lot of different people. They all said no. Then when I went to the bank, I hit pay dirt.
At first, I wasn't that interested in that job. And when they told me what my salary would be, I realized that I'd really hit pay dirt.
One of my fondest memories was searching through my grandfather's attic when I was a kid, convinced that some day I would hit pay dirt.
2. Expression：make a killing
Explanation：To have a great success, especially in making money. （获得成功，挣大钱）
Example：Since you're going to make a killing, I guess lunch is on you.
If word gets out of our intentions, the rascals now in power may well seize the opportunity to make a final killing!
We can make a killing on these toys if we're able to market them in time for the holidays.
It's very rare for someone to make a killing on their first investment—it takes a lot of time and patience.