Electric drivetrains are much simpler, much lighter and much less costly than internal combustion drivetrains of similar power. Tesla's drive units consist of just an inverter, induction motor and single-speed reduction gearbox with differential. The Model S drive unit, for instance, is so compact that it fits entirely within the rear suspension assembly.
In mechanical terms, Tesla's drive unit is what one would get if the starter motor of an ICE drivetrain were mated with the differential, and everything else - the engine, transmission, drive shaft - were thrown away.
The complicated part is the inverter which changes the DC voltage of the battery into AC voltage of varying amplitude and frequency that operates the motor. Much like a mechanical transmission adjusts the ratio between speed and torque, the inverter adjusts the ratio of voltage to current. Low voltage and high current produce high motor torque at low speeds while higher voltage and lower current result in lower motor torque and higher speeds.
The cost, size, performance and efficiency of the inverter depends largely on the power switching transistors, and these are improving at a semiconductor pace. New silicon carbide and gallium nitride power transistors switch faster and operate at higher temperatures than silicon transistors. Faster switching reduces the size of filtering components and supports higher motor speeds, that in turn allow smaller, lighter motors. High temperature operation allows simpler mechanical design for cooling within the inverter. The result is that electric drivetrains are getting smaller, lighter and cheaper, quickly.
An indication of the weight and cost of electric drive systems can be found in this DOE presentation. Note (slide 5, bottom) that a 2010 development program at GM achieved the projected 2015 weight target, suggesting that the technology was even then 5 years ahead of DOE's optimistic expectations. Our cost and weight estimates for Model 3 drivetrains is based on this DOE data, extrapolated forward to 2017. Cost estimates for the high performance drivetrain versions used in the 366PD car include a substantial premium for "high spec" electronic components, high performance magnetic materials in the motor and presumably low production rates.