The motor is 4.5A and 0.71 Ohms unipolar. You have a 36VDC supply.
1) The motor "takes care" of 3.2V (4.5A times 0.71 Ohms) so that leaves 32.8V (36V - 3.2V) you have to drop.
2) 32.8V divided by 4.5A is 7.3 Ohms. You will need two of them.
3) 32.8V times 4.5A is 148 Watts. Two resistors are needed per motor so you will be dissipating nearly 300 Watts of heat per motor in these resistors.
4) Big resistors are expensive and $80 doesn't sound unreasonable for the size parts necessary to dissipate this much heat. For your trouble you'll get fairly poor performance (10:1 voltage overdrive ratio, full or half-step resolution, resonances, etc.).