Cost- - this is essentially a learning project for me, I agreed to do it for my old High School for free if they paid for the parts. The entire project had to come in at under 2k and of course I left about $500 wiggle room but that is not enough to get new or used CAM encoders from what I've found.
I think the best bet is to use the M91 as the I/o slave over CAN. I will look into that further.
I did find these...
Which will make the encoders CANopen compatible but at $200 each they are still not cheap and its just one more thing to program, power, and implement. If you think these would work way better than just using a M91 though...