| It does depend on the condition as to what its worth, and as the old saying goes, its only worth what people are willing to pay!
The problem is that people either tend to go the newish route (ie, the latest, most expensive model THEY can afford, be it a 996/998/999 etc) or go a bit 'retro' and try to find a mint version of an older model, such as the SP (be it a 851/888/916 etc)...a lot of people seem to be going the route of older models recently.
But if its not mint, then the price if effected quite dramatically I think. I bought my mint 1995 SP for 5K. I would say, if the bike was standard then this would be a good price for a minter. However, mine did also come with loads of goodies (mag wheels, slipper clutch etc.)...so I did get a bargain. I have no doubt if I wanted to sell it in a year or two, I could get quite a bit more for it than that.
I have however seen other 916SP bikes on here advertised for less, and not sell, mainly because of their condition I guess...so...to answer your question, although it is a great bike and is a collectable item, unless it is in really good condition, or can be restored to such fairly easily, I think he would struggle to sell it privately for more than 4 -4.5K. Then again...there are always people out there looking for a collectable item who would be willing to bite his arm off?!?!?
But if you are one of those guys who will probably buy it, keep it for a while and treat it well and get it back upto a good condition over time, then the price will not drop that much more, and in fact will probably go up over time.
But all of this talk of money distracts us from the most important factor...take the thing for a test ride. If it is a true SP, then it will blow your socks off, especially when compared to the 851 you have been used to.
Race Cams, Twin injectors, Big valves, Titanium con rods, 50mm full system and a 11,500 rpm rev limit...the thing has to be ridden to be appreciated.
If you want the 916 shape in any of its guises, then In my opinion...your going to struggle to find better.  | 
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