Q: What is the efficiency of a stepper motor driver? 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? 

A: A stepper motor driver is more or less a DC/DC converter with variable voltage output. So it's probably something around 90%. The same efficiency as a regular DC/DC converter. 

A: 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. 

A: 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.
