How to Improve Inductance in a Stepper Motor?

Dear Sir, I have developed a stepper motor that is used in energy meter for registering the energy consumed. The core that I have used is CRGO lamination. The wound copper wire to generate inductance of 500 mH (imported ones have 1.2 H !). When we supply pulsed (+ & -) power to the motor, it rotates by 180 degrees in clock wise direction. The pulse width is 40 ms of 5V amplitude. The motor is supposed to operate from 3.0 to 5.0 volts amplitude. When, we reduce the power supply to say 4.0, the motor start jerking and does not rotate. In order to improve the magnetic power we have to go for more no. of turn of copper wire or use better core like permalloy. The cost of motor goes up. The present 5.0 volt operating motor costs 0.08 usd. The cost can be increased to maximum of 0.1 usd. Tried using low carbon steel as core material, a mariginal improvement in inductance found (about 600 mH). Please suggest any means for improving the performance of stepper motor. Thanking you, Sheshadri, India