Wholesale is around $10K for very good condition. Mine had the check engine & airbag lights on, and the horn had quit working. It also had an oil leak and bald tires that would not pass inspection. Despite all this the car was in very sound mechanical condition, drove great, and probably needed around $1500-$2000 put into it to be in good resale condition. I took $8500 but could probably have gotten $13 - $15k if I'd fixed everything and replaced the tires. I just was not wanting to do all that and then have to sell the car as well. But yes, one can definately get more by not going the trade-in route.
I think most buyers of new Jags consider the time and hassle element and opt to do the trade in. The new XJ8L sticker price was $67,535 with premium Alpine sound, bluetooth, and seat/steering wheel heaters (we REALLY need those seat heaters down here in Texas). I ended up trading my '99VDP and writing a check for $53,500 to drive the new Jag out the dealership's door free and clear with tax, title & license included - it was on the showroom floor where they put the really pretty ones. I'll be interested in seeing how much they ask for my old VDP, which is a really great and cool looking car.