It is of the order of 1 m^−3 (intergalactic medium) to 10^30 m^-3 (stellar core). My calculations show that it should be of the order of 10^23 m^-3. Is my order of magnitude alright?
I recommend you to look through the paper "A two-temperature accretion disk model for Cygnus X-1 - Structure and spectrum" of Shapiro, S. L.; Lightman, A. P.; Eardley, D. M. (http://adsabs.harvard.edu/abs/1976ApJ...204..187S).