I went through this a few years ago (though i had no desire of buying the car back) but i challenged their "comparables"
(That would be the cars for sale in your area with similar options and mileage)
And found several comparables that were a little farther away but closer in options
Once i submitted those, they agreed with me
(I cant find the numbers now, but they adjusted it up several thousand dollars, and that was on an 08 charger, awd 160k)
If you are a good receipt keeper you may be able to prove a higher value based on that too
The one thing to keep in mind is, this is not a final offer, you are not obliged to take it