Dead thread response:
All depends on how you collect the data to generate the variables, how complex the variable rtns are, and what sort of motion effects you want from the steppers - synched-steps, rampy, constant dance, but...
Arduinos. One master, multiple slaves on the I2C bus. Each slave (uno, nano, mini, micro, etc) could drive 1 - 7 (?) steppers with 'effects' if needed. Do the big calcs on the master, send out the new postions, let the slaves do the drudge work of spinning the steppers and maybe acknowledging they rcvd and executed. 9 bucks for an Uno, under $4 each for nanos, minis, $2 ~$5 per driver
You could also use just 1 DUE (54 dig i/o), but it's a 3.3v board...