Fully diluted, there are 15.3M shares outstanding. In a buyout scenario the PPS would be a on a fully diluted basis as obviously all the stock options and warrants would be exercised. From there, it's simple math:
$300M buyout / 15.3M shares = $19.61 price per share
$500M buyout / 15.3M shares = $32.68 price per share
$1B buyout / 15.3M shares = $65.36 price per share
As you can see, the numbers are pretty mind blowing, given the fact that that this drug CONSERVATIVELY should do $500M per year in peak annual sales. The average biotech acquisition is between 3-4x peak annual sales. Seems unlikely that they'd fetch a number in the billions before approval, but it's very easy to envision a buyout shortly before or after the NDA is filed for somewhere in the $300M - $1B range.