Something else I would take into consideration is the kind of lens it is:
On an ultra wide angle a scratch like that might be quite problematic (ie have a noticeable effect on the images).
On a telephoto it is likely to have no effect whatsoever, other than an immeasurable reduction in contrast, perhaps).
Also, a scratch on the rear element is much more likely to cause problems than one on the front (more likely to be in greater focus), again relating more to a shorter focal length.
I don't have experience of lenses with scratches on the rear element, only on the front, as follows (all prime Nikkor lenses):
35mm f2: looked like an absolute dog with fine surface scratches all over the front element: never noticed any issues whatsoever, compared to the pristine version I owned later
85mm f2: slightly better looking but still lots of fine scratches (from cleaning): never noticed any issues whatsoever
180mm f2.8: one big scratch on front element (similar to the one you describe): never noticed any issues whatsoever (what a bloody sharp and contrasty lens this is, by the way).
Don't know if that helps.
However, in terms of negotiating with the seller, I would ask for something like 25% off and then maybe settle a little lower.