Likely demographics for such a project would likely skew young (Gen X and younger), as that is the target audience for the horror genre, and Purge films in particular. It would also include horror/thriller fans that would consume a Purge-style story regardless of its focus as well as fans of both Jane Austen and the horror genre.
That being said, there are some considerations that must be taken into account. The films in the Purge franchise have grossed nearly $447 million on a budget of $35 million combined. However, the film Pride, Prejudice, and Zombies, a parody horror satire based on Austen's original Pride and Prejudice grossed only $16.4 million, and the budget was $28 million (in spite of having a 92% favorability rating from viewers). Of course, the Pride, Prejudice, and Zombies book was a tremendous hit, making it to the New York Times Bestseller List and inspiring a prequel novel (not to mention spinoffs Abraham Lincoln, Vampire Hunter and Sense, Sensibility, and Sea Monsters).
There is demonstrated success for these types of crossover stories, but it would seem that if the story skewed more Jane Austen than Purge, it would perform better in written form. However, if it were a Purge franchise film with a Jane Austen-inspired focus, it would almost certainly be a tremendous hit.