Question- How much should the lump affect the price?
Perhaps a better question would be: How much should you pay for it after
factoring in the cost of obtaining and installing a XK powerplant with
gearbox (if needed) and then comparing that to current market prices?
For me an E without the XK is not an XK-E. The car you are considering
is a V6-E, a different animal altogether.
Cheers,
Will