The S-Pen itself is $30, that plus a larger display and whatever other improvements will make it worth $100 for a lot of people. The only reason they wouldn't use an upgraded SoC is if they're releasing the device before the new chips release - which they are. The RAM, Camera, etc - these are all things that get updated when updates are available, not just to fill a spec sheet. We're looking for performance, not just meaningless numbers. If the dual camera rumor is true though, then that + S-Pen is $100 right there.
I seriously doubt the S9 will have 128 GB as a base storage option, haven't seen anything to indicate they were going to change cameras or increase RAM, etc. The only thing that seems somewhat solid is that they'll be using the Snapdragon 845, which will be similarly priced to the 835 so no price increase to pass to consumers there.
Finally, the Note 8 is not meant to be an upgrade from the S8. It's meant to be a larger, s-pen enabled option for people who want the Note line. The idea isn't supposed to be that you buy the S8 and then 6 months later buy the Note 8 and then 6 months later buy the S9. Samsung still believes the average usage of a device is between 1 and 2 years.
The Note line was meant for one audience while the S line was meant for another audience. After that GS6, that pretty much changed and the Note line has basically become the S line with a stylus. $30 extra for the stylus or not, it lost its identity and the different costumer base it aimed for. A dual camera would be an interesting change, probably something in line with the addition of the Iris scanner on the Note 7 last year, but these changes are really limited when you compare it to the updates the Note line use to get. Lets say it's all 64GB
We know it won't have the S845, but Samsung could have put in some work to use a newer and better Exynos chip.
The S line was aimed at one group of buyers and the Note line was aimed at another group. So the cycle of upgrades was 1 or 2 years. Even so, Samsung use to actually provide improvements in the Note line over the S line released that same year.
In 2013, the GS4 had the S600 while the Note 3 had the S800 SoC. GS4 maxed out at 1080p 30fps video recording and playback, while the Note 3 had 1080p at 60fps and introduced 4k recording at 30fps. 2GB of RAM vs 3GB RAM. VoLTE,
Then it was the GS5 with the S801, 2GB RAM, improved display quality over the Note 3, questionable ISO Cell Camera imo, but at least they tried something new and water resistant. The Note 4 came with the S805, 3GB RAM, a 1440p display, another new sony camera sensor, and some other changes.
Then came in the GS6 and the design centric nature we are in.
There were a lot of changes made on a hardware level between the S Line and the Note Line and the S. Now the Note line ends up being an S line version with a Pen, while the S line gets all the upgrades. It's a trend I hope Samsung changes.