The asking price for the Pentax version of that lens is around $200. I don't know if they are getting it. A few years ago, Cameta camera (on ebay) dumped a bunch of copies at $99 each. I don't know the reason for that and had actually ordered one. Never got it though as I changed the order to the Tamron 70-300 (which I just sold for about half what I paid for it). That's neither here nor there. I can say that the people buying the Sigma at the time, were quite happy with the photos they were getting. If I remember correctly, they suffered in large numbers from loss of the ability to Autofocus. Again, don't know why, as the AF is screw drive, perhaps a weak link in the lens broke. Zoom creep was also a problem that many had to endure with this lens. Optically however, it seemed a good performer on the 6-10 Mp cameras of the day. That's the Pentax mount versions. I know nothing of the history of the Nikon version.
I would start it at around $100 on ebay and let it go for whatever it sells for, but that's just me. Or if your BIL actually Paid for the lens, start it at whatever he paid.