I hardly suspect it was financial reasons; Mike's show was the cheapest to produce weekday show on the lineup. He and his wife/producer combined barely made more than the low six-figures. Perhaps it was the late time slot, and/or his opinions that was his undoing.
This was extremely unprofessional of Air America, firing a host half an hour before airtime (filling in for Randi Rhodes), and not giving him a chance to say goodbye.
I suspect that "financial reasons" referred to lack of clearances and advertising, not compensation to the Malloys. Not many stations carried Malloy or, if they did, delayed him until after midnight (KTLK in Los Angeles runs a local show from 10 pm to 1 am, followed by Malloy). And, as others have pointed out, you rarely get a chance to say goodbye in this business (unless you're retiring or [maybe] leaving for a better job while still in the good graces of management).