Look on goobike.com (Jap bike trader type website) to do some research, they have around 100 NSR SP's listed (+ many more R and SE's), of varying age, mileage, condition. You can get a feel for price range in Japan, which seems to have some broad parallels with the UK market, if you don't consider some recent crazy UK dealer prices. Also search this forum, there have been several recent topics on UK prices, you'll find them over the first 5 discussion pages.
Try using google to search for the goobike website, then you can get a partial translation to english.
As per Maxim's note, many have been on their side at some point, mine had although I didn't discover this until after purchase. Just have a good check to ensure it runs 'straight', no ripples or creases on the frame, around the headstock etc etc.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30