IMO:
Japan
It's been doing between 10-20k each week although it's been declining. Seeing as the comparitive cost of the 20Gb PS3 in Japan is around $400, the new model would need to come in under this amount, even $350, if this is the case, then I reckon it could be closer to double.
NA
Since the July price drop to $499, the PS3 has been doing 20-40k each week, again it's been declining. A new price point at $100 less would bump up the figures at least double I think. I don't see any reason why it can't be doing 90-100k+ each week. Although it's likely to be closer to 70k each week.
Europe & Others
This is where I reckon the biggest impact will be. We are currently seeing around 30k a week. A £299 price point in the UK would bump sales up to Wii numbers i.e. 70-80k.
The bottom line is, IF this happens, it should have a huge impact on the sales for Xmas. But it will be a shame if more and more PS3 games get delayed until next year seeing as the PS3 needs good AAA games for Xmas and the list is getting smaller all the time.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)