Transmission distance rs485 vs ethernet
but i cannot understand why the difference between distance and baudrate of ethernet and rs485 is more than a factor 10 (ethernet has an actual baudrate of 125mbps due to 4b5b encoding)


Well, Ethernet (you mention rates specific to what is called Fast Ethernet, by the way, not just Ethernet) demands better cables, and the transceivers are technically way more complex.

It's like a high-speed train goes faster than a 1960's tractor, but then again, the high-speed train is a tiny bit more complex, and needs rails.


  endgame for me would be to create an 2 wire (half-duplex) master slave (rs485-like )network with around 200 nodes , a length of 1000 meters and a speed of ~10 mbps.


Network. You say it yourself: Network. Use a networking standard that already solves all layer 2 (and above!) problems for you – RS485 really only defines how bits are exchanged between two ends of a cable. You need to come up with a working scheme to connect 200 nodes, and how to deal with collisions, and how to address nodes and …

Use ethernet. This is exactly what ethernet was built for: building medium-to-large sized networks. Switches are cheap, and reliable, and solve all problems of collisions. Ethernet offers tree-spanning, automatic discovery, switches do loop detection, support for it is available for every computer and many, many many microcontrollers…

100m work without any ado with standard Cat5 cabling (which is cheap as hell), and 1000m would be easy to build using cheap consumer switches as repeaters, or, professionally, with a media exchange to optical. 


  and theoretical if 100base-tx would run at half speed would the usable distance then muliply with 2?


No. That's not how things work: The distance is limited by drive strength; while that also defines speed, there's simply no "half-speed mode" you could use.