battery not charging on 1998 Cadillac DeVille

car would not turnover. had to get new battery. Warning light still came on saying Battery not charging. Had test done at local Advance Auto to see what was draining battery.
Was told some light or something is on and would drain battery slowly if not used. Driving car and battery went completely dead. Had to have it towed. Now they say it is the alternator. Is this costly repair.

Asked by for the 1998 Cadillac DeVille
It sounds to me like it was the alternator on your 1998 Cadillac DeVille from the very beginning. The battery not charging message would generally mean that the alternator is not working correctly.