Well, snap. Stuff like this is making me reconsider.
Yes, an internal SSD with enough storage is still pretty expensive, but the prices of those things have been going down considerably. Heck I remember upgrading my 40GB PS3 to a 160GB HDD and it cost me just as much as my curretn 500GB SSD in my laptop. So it's an investment to be made not right now, in this November, but perhaps a year from now or so, maybe even sooner (no idea how fast/slow SSDs are coming down in price).
But damn man... SATA3+SSD speeds on my PS4? Amazing. Would be especially great for games with (currently) long loading speeds. GTA5, Fallout 4 and Bloodborne come to mind (and these are games that generally have 1 single big load at the start too, with exceptions here and there).
On the other hand...eh... still not sure if the Pro is a decent enough upgrade. I mean, yeah it's more powerful but being limited by the OG PS4 makes me doubt it a bit. If it were a brand new console (no strings attached) I'd be more at ease with it (though if this continuous upgrade system proves successful I doubt any new console won't be attached to it's direct predecessor).
PS:
FOR PEOPLE WANTING TO GET A BIG SSD DAY 1
Wait for Black Friday+Cyber Monday. Browse stuff like Amazon and Newegg, they usually have at least 1 awesome deal on SSD's, depending on how far you are willing to go with your money (though in any scenario they'll be cheaper than on regular pricing).