Is there a fieldbus with more than 250 nodes per segment?
Your main problem will be data traffic. If you have 250 nodes that should report in with 10Hz, then that's 2500 messages per second. Your main focus needs to be to keep the data traffic down. Maybe you can create sub-systems, maybe you can send an average at lower frequencies etc.

At a glance, it would seem that CAN is the ideal solution for this. It is rugged, it is real-time, you can get fairly long distances. If including supply, you can do with minimum 4 wires. That is, depending on how much current the nodes draw - sharing supply ground with signal ground is a bit of a dirty solution.

However, since your timing requirements are so high, you'll be looking at the higher baudrates. And you want large distances. Which in turn means that dedicated signal ground and cable shield might be needed. So your specification is contradicting itself: you can't get "fast", "good", and "cheap" at the same time. 

In addition, there's not really enough info here to draw any conclusions. I suspect that your specification needs to be revised before you run off to pick a fieldbus.

As for the higher layer fieldbus protocols: unless you need to be compatible with some industry standard computer/PLC, there's no apparent need for CANopen or Profibus etc. These add lots of complexity you can do without, and most are restricted to 127 nodes (for good reasons). If this is a closed system, then a custom CAN protocol might be the ideal solution. 

Profibus and MODBUS are also quite old technologies, I wouldn't recommend using them for new design.