I am interested in the comparison between the Plasma Membrane Calcium ATPase (PMCA) and the Sodium-Calcium Exchanger (NCX) which are two pumps on the plasma membrane of cells that serve to move calcium out of the cell and maintain relatively low resting calcium concentrations. Typically, I see the statement that the PMCA has high affinity for calcium but a low pumping capacity, whereas the NCX has a low affinity for calcium but a high pumping capacity.
In the literature, I can find relatively simple models of the PMCA as well as data of estimated dissociation constants for it. However, I am having a much harder time finding the concentration of calcium where the NCX becomes relevant. It might be because the NCX also depends on sodium concentrations, membrane potential, etc., which gives rise to much more complicated models. However, there must be some research that backs up the "NCX has a low affinity" claim.
So, what is the typical calcium concentration range where the NCX becomes relevant, such as a calcium dissociation constant where the pump is at 50% of its maximum capacity?