I may be mistaken but I don't think that the first three receive a salary.  People are willing to pay to hear them and their rates are based on a straightforward supply-demand curve.  The Director-level pay seems about right, in my experience, for directors across various industries.  The IH Radio seems more akin to the pay of a Sr. Analyst or low-level manager, however, but maybe its a small organization.
I'm not a Donohue fan so I'll not comment because of that bias.  
I have to admit I don't listen to them so I won't dispute your "good investment" characterization.  I've heard that from the evangelical side (Benny Hinn, et al.) so I suppose it's possible.
That said, they are not "making money from religion" but from their arguments about religion.  While they may or may not be catechists, they are typically apologists - an entirely different career path.  The catechist simply teaches the faith while the apologist must explain and defend it before both friendly and hostile audiences.