How computationally expensive is generating a random number in C++?

The method that I am considering is from: http://ift.tt/2a83FKw

#include <random>

std::random_device rd;     // only used once to initialise (seed) engine
std::mt19937 rng(rd());    // random-number engine used (Mersenne-Twister in this case)
std::uniform_int_distribution<int> uni(min,max); // guaranteed unbiased

auto random_integer = uni(rng);

I'm also willing to use the rand() approach with srand(time(NULL)).

My question: how expensive are these approaches? Is one much faster than the other?