We are using this stepper motor driver (https://www.sparkfun.com/products/12779). In its datasheet, there is no mention of efficiency? How much is the efficiency in motor drivers like these?
In motion control efficiency is usually mechanical output power (w*T) divided by electrical input power (U/I). Obviously it is different for different working points. For a standing stepper it's zero. On higher speeds sometimes nice figures may be reached, but bot very nice. Because for stepper you always take spares, otherwise it slips.
This is the stepper motor linked in the question: -
How much is the efficiency in motor drivers like these?
It's not great. Consider the saturation voltage of the output transistors in the A3967 chip that drives the stepper: -
If you are taking the full 750 mA into your stepper then a total volt drop of about 3 volts can be expected. If you are running from a supply of (say) 15 volts, you are going to only see about 12 volts across the stepper motor coils. This means that peak input power is 15 x 0.75 amps = 11.25 watts whereas peak output power is only 9 watts. That means the peak efficiency is about 80%. If the supply voltage is (say) only 6 volts then nearly half that voltage is lost in the driver rendering an efficiency of only 50%.
There are better devices that only drop about 0.5 volts. See this for a critique of other similar basic stepper drives / motor drives for a comparison.