From what I've heard, if you trade in your Model S to Tesla, they use an equation thats rumored to be:
$1 per mile + $1,000 per month of ownership
so take my Model S as an example, i paid about $90K for it.
it has 21,000 miles on it, and I took delivery January 2013 (21 months ago)
$90K original price
-$21K mileage
-$21K time (months)
~$48K trade in for a Model X, and thats if it happened today.
In other words, I think you can do better with a private sale, than a trade in to Tesla.
the issue is, what would you drive for another year?
plus, I went through early adopter issues with the Model S , I think waiting a year or two, for Tesla to "get the issues" squared away would be prudent.