There is no rhyme or reason to twin tub pricing. Some units in great conditon go cheap, others in less perfect shape can sell for very dear amounts.
Normally much depends, as with all large appliances, on location. If the unit is some where out on the moors or some tiny village in Devon, it probably won't sell for the same amount if it was some where nice and central, say London, for instance.
However it is worth a trip, if possible to nab a unit in good or even mint condtion. Some have driven and taken a boat train and reverse to pick up good units on the other side of the Channel.
Your other option is to keep your eyes open at tips and curbsides for discarded units. You would be surprised how many perfectly working twinnies are chucked out simply because someone purchased something new and modern, or their gran passed on and no one wanted the thing.
When purchasing a twin tub, first inquiry to any seller should be about the pump. Hoover models in particular tended to have pumps that gave out, which wasn't a problem back in the day, as the pumps were designed to be repaired. However Hoover no longer produces twin tubs, and stopped recently stopped production of spares as well in the UK. While there are some "new old stock" parts and bits about, pumps are hard to come by. Without the pump, the unit is useless as no other pumps will fit.
L.