Subaru Outback mileage -- what's a good cost deduction for mileage?

There is no significant difference between those three mileages. Evaluate on other things like condition, options etc. You can always offer less!

Your looking at the wrong issues. The difference between 50K and 56K miles has nothing to do with the value of the vehicle. How was the car driven during it's life, has the regular maintenance been performed? What is the overall condition of the vehicle, options. Unless you know the personal driving habits of the original owners, mileage is insignificant. Look well beyond the odometer, ask for service records.

That said, the quick wholesale delta is $1k/10k mi.