We purchased a stair lift earlier this year for my mun in law. The cost new is around £1800 fitted.
The lift was used for less tham 3 months unfortunately. Companies offer to buy back lifts, so I enquired as to the price offered.
Around £200 seems to be the going rate.
One company did say that if they didn't purchase the lift because it turned out to be over two years old, they would charge £150 for the engineers visit. (No doubt plus vat.)