at 45k, they would at least be able to cover what you would want for yours.
in their systems all they'll see is that any 15 xj's are roughly 31k with the 3.0L out there. R and the options on that should make it more interesting and higher value. even so, mid 30's would likely be all they should honestly hope for. but some merican would likely pay pretty well for it and their dollar means close to 40 for a no muss, no fuss sale.
the issue is that they will also see that the only 04 is300 that has been on the market or sold recently *just* sold at peel chrysler and had been asking 3500 with 290km.
basically, you can't get it from both sides. they want to make a bit on both ends, and you trying to get over retail for yours means they need over retail for theirs to have it make sense.