Hello Germain38
There are several differences which justify this price difference. You can find more information on our homepage https://www.irobot.ch
Compared to the next lower priced product, the Roomba i7+, there are the following features:
New 3-stage cleaning system with Powerboost.
- Our most powerful suction (40x vs. Roomba 600 and 4x vs. Roomba i7+)
- Corner brush
- 30% wider rubber brush
- Anti-Allergen System
- PerfectEdge™ Technology
Roomba s9+ can also clean a larger area in less time.
And of course there are several differences compared to the Roomba 900-, e-, 600- series which Pit.hostettler has already explained to a large extent.
Many greetings
Your iRobot Team

Smart maps are great. You can define zones where it should not clean and you can divide large rooms into virtual rooms and only clean a part of them. The older Roomba 980 does not have Smart Maps.
With the + you also get the hardware to empty the robot automatically. I find that very practical. Emptying the Roomba 980 left more of a mess (dust around) ... so it wasn't that practical. The S9+ automatically empties the robot. Disadvantage: You have to buy the emptying bag.