Is there such an expression in German: gemachter Mann? Is there any other meaning to it but well-off man? How is it used?
As far as I can tell, it means successful, prospered.
Sie haben sich gemacht, "You improved, you made something of yourself"
sich heraus machen, like herausputzen, "to dress fancy", cp. "make-up"
dialectal to go somewhere, e.g. nach drüben machen, "to go over there" (chiefly from the GDR to the BRD), mach dich vom Acker, "take a hike", perhaps cp. to have come a long way
A cross polination from mögen might be possible, i.e. ein gemochter Mann, "a well liked person", just like any other non-obvious derivation.
Compare ein gestandener Mann, "a well aged mann", seinen Mann stehen "to prove oneself", ein angesehener Mann, "a well regarded man", and perhaps Stand, "standing", Ständegesellschaft "rank-society", Ansehen, "respect".