And it makes sense for them to assume the show will sell well, especally if it gets good reviews (which we don't know if it will as the show isn't open).
Take a look at The Producers. A month after the show started performances in New York, they raised the price to $100. This was the highest ticket price in New York at the time. And the show still did VERY well. Soon after, the top price was raised to $480. If Young Frankenstein gets the same type of reviews as The Producers, it's pretty safe to assume it will sell very well.
That is why they are setting these ticket prices so high.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.