I think you did well. This is the way I would look at it. You received a new (maybe nicer, I like the interior and mygig on the '08s) car and gave them a car with 24,000 miles.
$6,000 / 24,000 miles= $0.25 a mile, not a bad deal.
If you kept the car 18 months (estimate) it only cost $333/ month ($6,000/18).
To put 24,000 miles on an SRT8 in 18 months and it only cost $333 a month.
I consider that an awesome deal.