That looks like a really old lens - I can't find any mention of any Sigma 300 2.8's prior to their current black ones. If you could post a closer pic so we could get the various acronyms to go by as well we may be able to help. I'd doubt that this is a particularly strong lens simply because very few old Sigmas were, but the sheer amount of glass would probably mean it would fetch a few hundred pounds as a guess - it's always hard to value older or rarer lenses, particularly with very little detail.
Assuming it's EF mount (unlikely tbh), with Canon it probably won't work to autofocus it - the chip is probably out of date which will mean new cameras cannot autofocus it and there may be other errors, and there's no mention of it on the Sigma website so I doubt very much that they'll still produce parts to supply and service it.
The extra black tube is a lens hood to cut out stray light rays causing flares/ghosting, and for protection.
