Trade in values are always lower than the websites or private party. You already know that, otherwise you'd sell it yourself. Dealers base their price on the average auction value.
You parents X5 is going to be hauled off to auction and sold. The dealer is just hoping to break even.
It's possible the used car manager may see your X5 and want it, but guess what he's not going to pay more than? That's right, auction value.
In the end, the dealer is going to maintain their margins. If you want a high trade in value, you'll pay more for the car and have higher financing costs.
If you want the best deal, sell the X5 yourself and negotiate the lowest price for the Lexus.
The other problem is you're not a serious buyer. Hiding behind a computer screen telling other people how much they should pay you for a vehicle they've never seen is cowardly. What you'll find if you go visit these three dealerships is that the deals are all going to be very similar in the end.