I agree, ICHTB. As I said upthread, 'might need underpinning in the future' could mean anything.
I would get a full structural survey and see what the problem was. Expecting to get a 50% discount is ridiculous, unless the house was in a state of collapse. Any house can subside, or there would be no need to insure against it happening, and any house 'might need underpinning in the future'.
Much depends on what 'has had subsidence in the past' means, too - as has been said, drains, trees, all sorts of things can cause houses to move, but without knowing the cause and any remedial action, it would be bonkers to make a decision if the house is otherwise ideal.