too cheap could mean loadsa hidden issues to spend time/money fixing and they're trying to sell it fast, or the seller doesn't value the car.
too £££ and the seller is ambitious in ripping u off.
ppl say u get what u pay for, and arming urself with more knowledge bout what to look for is an essential investment to reduce the risks of being ripped off but yea there's a certain average market value.
purely imo of what I'd pay,
a low spec 1.3 pre-face is bout £100-400
a high spec 1.3 pre-face like superS or SLX is bout £400-600
a high spec facelift 1.3 runs £600+
very low milage & showroom condition obviously ramps up cost alot into £1k+ but be wary of cars that's been standing around long time out in the elements cos all cars with ferrous metals corrode inside & out (unless it's been stored in a hermetically sealed container

)
a stale non-serviced car could just be as problematic (rusted or seized internals, brakes, cooling system etc) as a high mileage but well serviced car.