Sigma has been heavily discounting this lens for a number of years, now. I bought it about five years ago, new, for a price just shy of $600.
It was a great lens but it rarely made it into my bag since it was such a tank.
Also, I was a bit disappointed with the performance off-center. I had the Sigma 40mm f1.4, which was a stunning performer (even by today's standards) across the frame. The 28mm was great in the center, but was not super-sharp near the corners and showed a lot of astigmatism.
Below are LensRental MTF charts that demonstrate what I am talking about.