Hi Ildiko , the price you paid for the car is not relevent . What does matter is the model and condition of the car. I agree with mic that vehicles sold within clubs do tend to sell at more reasonable prices as opposed to crazy prices that some speculators will try. The thing is that "reasonable" is a variable especialy as the 126 has now firmly moved into the classic car market. In right hand drive models they are much rarer than the higher priced Fiat 500s. There will always be "bargain" cars out there and a few will be genuine but many of the LHD imports have MOTs where the car has not even been to the garage to be checked. So I would value an average , on the road RHD aircooled 126 at £800 to £1.500 then upward from there for special models , low mileage , rust free , etc , etc.
Top end seems to be about £5,000 for an exceptional car and I think there are a few Bis owners out there that would tell you where to go if you only offered them £2,500 for their cars