The proton gradient is stored between the mitochondrial double membrane. More protons mean more hydronium ions and lower pH, but how much exactly?
I agree with @alina that it is well accepted that the pH difference across the inner membrane is ~0.5-1 units. However, the absolute value of the pH in the IMS will vary between organisms and even cells. It is experimentally difficult to measure it, therefore most of the times only the matrix pH has been reported. Using the output from the BioNumbers database as an example:
human endothelial cells ECV304: 6.88 (IMS), 7.78 (matrix)
human HeLa cells: 7.98 (matrix)
rat cardiomyocytes: 7.91 (matrix)
According to the authorities Roos and Boron the difference of pH across the inner-membrane of mitochondrion is 0.5 with pH outside the membrane equal 7.0 and inside the pH is 7.5 (pH-mit = 7.5, pH-out = 7.0) according to Intracellular pH