We take ours through a car wash that does the vacuuming and wipe down after, etc. The customers love getting it back smelling and looking clean...and for me that's 12$ well spent.
Our department gets nothing from the sale of a vehicle at auction, so there's little incentive for us to detail it. I remove the emergency equipment and we turn over the keys to the property department. There were years I was sending Crown Vics with no headlights, tail lights or any other part I felt could be used as a spare for the rest of our fleet.
We also tend to run cars until they're on their last leg. Then I'll swap out the blue for amber and assign it to our student CSO program until they kill it. Then it can go to auction.