  Einstein-Podolsky-Rosen steering is a form of quantum nonlocality which is
weaker than Bell nonlocality, but stronger than entanglement. Here we present a
method to check Einstein-Podolsky-Rosen steering in the scenario where the
steering party performs two black-box measurements and the trusted party
performs projective qubit measurements corresponding to two arbitrary mutually
unbiased bases. This method is based on decomposing the measurement
correlations in terms of extremal boxes of the steering scenario. In this
context, we propose a measure of steerability called steering cost. We show
that our steering cost is a convex steering monotone. We illustrate our method
to check steerability with two families of measurement correlations and find
out their steering cost.
