FYI - don't buy a used Tesla from a non-Tesla dealer. Tesla has their own CPO/extended warranty programs at NO extra cost. You WILL want a warranty because NO one is fixing/servicing these cars yet.
Average used Tesla S is about $30k-$40k and comes with a pretty amazing CPO warranty.
As an aside though... hold out until summer 2020. There's going to be A LOT of electric car releases which will drive down Tesla demand... and ultimately pricing... new and used.
EDIT - enjoy!
https://www.tesla.com/inventory/used/ms