Remember, you cannot shut down the fabs and lay people off... those would be the terms of the agreement. That's $1.5B of losses per year, which, even with $1B given by IBM, would lead to $0.5B of loss in the first year. The second year would be, again, around $1B of losses... I wish I had $1.5B to burn for the next 2 years :-)
I heard the split happened when IBM licensed the process it developed with huge amounts of GloFo investment to UMC sometime back. That's when GloFo moved a large number of its employees out of Fishkill and started developing processes on its own. This hurt both GloFo and IBM - GloFo struggled to get 20nm to yield on its own, leading to licensing a process from Samsung at 14nm. IBM's semiconductor division lost a valuable partner who was putting in money and resources - this coming at a time when they lost the game console business was a death knell.
Its amazing how that one decision from IBM management to license to GloFo's competitor cost them.