I wonder if this has anything to do with Tesla releasing Model 3 75/75D range numbers soon on the 28th. Otherwise, it doesn't make any sense to drop CPO prices now at the beginning of Q3 2017 instead of early September. If they didn't believe the Model 3 would affect Model S sales, they would have waited until September to maximize Q3 sales. Apparently, they do think Model S sales will be negatively affected. This might also explain why they are delaying AWD Model 3 production.
I'm expecting 285 mi EPA for the Model 3 75 and 297 mi EPA for the Model 3 75D. Also, I expect the Model 3 75 to start at $45K and the Model 3 75D at $49K. The Model S 90D has 294 mi EPA. Maybe Tesla thought the S90D CPO's wouldn't sell well after they announce the Model 3 range numbers.
Basically, the Model 3 75D should have the same range and performance as the Model S 90D. However, because the battery is 17% smaller, it means the Model 3 75D will cost 17% less to charge, battery degradation will be 17% slower because of fewer charge cycles, supercharge sessions will take 17% shorter because the car is 17% more efficient and of course, the Model 3 will cost much less not to mention it has the newer cell chemistry which might be a requirement for Supercharger V3.