Its impossible to put a value on it with so little info, if its a non runner its even harder. could be anything from a wrecked engine to a loose connection. If it is a non runner though it will obviously reduce its value and give you something to drive the asking price down with.

If it is an SP model it should have the following:
Dry clutch which will have either a green plastic vented cover on it or no cover and exposed clutch.
Magtek wheels which have a smooth rim with no lip on the edge and Magtek embosed on the spokes.
It should have a rear shock with a remote reservoir which attaches to the right hand frame spar.
Adjustable front forks with blue anodised knurled adjuster on the fork top.
Bodywork, if you are lucky enough to find it wearing its original bodywork it should have either Rothmans (94) HRC (95) or Repsol (96)
Subframe, it should have a sticker with model designation on the right subframe rail under the drivers seat, cant tell you what these are off the top of my head and subframes are easily changed so not a reliable method of identification.

SE model will have the remote reservoir rear shock and dry clutch, no magtek wheels and adjustable forks but with a slotted brass adjuster and no blue adjusters.

The R model will have wet clutch, no remote reservoir, non magtek wheels and forks with preload adjustment only.

All the 28's should have pgm 4 credit card ignition and clocks with tacho and digital speedo and temp gauge.

Study the model history on here and Jamies site, plenty of detail pictures of all the important parts to look at.

It could of course have any combination of parts as they are easily swapped between models,As a non runner I would check the card and ignition turn on and work, the engine turns over, you can turn the powervalves by hand (if they are seized they could have burnt out the pgm) Second hand pgm's run to a few hundred if you can find one.

Regarding what to pay, well if it is a 28 SP the wheels suspension and dry clutch assembly would be worth £1200 plus as parts so you can work from there. A lot of body panels are no longer available so good original bodywork is worth paying for, Standard original bikes fetch a premium so look for as many original parts as possible, if you are restoring to standard the bits and bobs all add up.

If you are going to view the bike take some pics and post them on here for some opinions.If its an SP I will put you off buying it and nip round there myself though