Like Nick said, advertising paid so much better than comics that once a comic artist crossed over he usually stayed in advertising. Nevertheless several comics people seem to have done one or two ads but remained in comics. I remember a comics-format toy ad back in the 60s (70s?) drawn by Chic Stone. Murphy Anderson did a sword-and-sandal movie poster. Didn't he also make drawings for the Major Matt Mason toys? I recall a toy soldier ad by Rocco Mastroserio. There was one by Russ Heath as well, wasn't there? I don't own the books any more so these examples off the top of my head may be wrong.
Storyboarding for commercials, TV shows, and movies is another place to which comics artists could escape. I believe Rafael Astarita became a storyboard artist. I read that ( sadly-underrated) pulp/comics artist Henry Sharp moved up to designing props for the original "Mission: Impossible" TV show.
From reading interviews with Silver Age artists in "Alter Ego" it appears that despite low page rates, a comics artist could make a decent middle-class living back then. He'd have to be good enough to appeal to editors and able to produce lots of pages month after month, but a number of artists seemed to have pulled it off.