SIAC, a subsidiary of NYSE Group Inc. that runs the systems for the New York Stock Exchange, is migrating to IBM servers running AIX. Feldman said the 1,650-MIPS mainframe that SIAC is moving off of processes data after trades are completed and generates reports for NYSE member companies. It also gathers data for regulatory purposes.

Feldman didn't disclose specific savings goals but said he expects the move to Unix systems to cut the costs of running the applications on the mainframe in half. He considered outsourcing the mainframe but said he also expects to gain business advantages through increased integration of the systems once the migration is completed.

Reader's Digest is mostly switching to Solaris-based systems from Sun Microsystems Inc., along with some AIX-based machines. Sementilli said he had concerns about outsourcing the mainframe, partly because that was the only system involved. As a result, he wanted to seek out a small vendor instead of going to a large IT services firm.

Infocrossing bills the publishing company based on usage metrics, such as the amount of disk space consumed and tape cartridges needed. Sementilli said Reader's Digest has cut costs by managing the data on the mainframe more efficiently. He added that the variable cost structure created an incentive "to look at our resources and see what we really needed."

"Once you're paying by the drink, every sip matters," said Peter Allen, managing director of Technology Partners International Inc., an outsourcing consultancy in The Woodlands, Texas. Allen said some CIOs are using variable cost models to control internal expenses through chargebacks.