Users planning to run Linux-based applications on IBM's zSeries mainframes, for example, should factor in the costs associated with getting enterprise-class support and maintenance services.
Depending on the kind of application and the number of CPUs Linux is running on, such costs could easily range from $25,000 (£16,000) to $30,000 (£19,200) a year, said David Mastrobattista, an analyst at Giga.
There's also the one-time cost associated with buying the zVM operating system - starting at around $45,000 (£28,850) - needed to run Linux on mainframe partitions. Optional zVM-related service and support cost about $11,000 (£7,000) per engine annually, Mastrobattista said.
On top of that there are software fees that users need to pay if they want to optimise Linux application performance on mainframes using workload management and systems management software.
"People need to go into this with their eyes wide open. It's not a no-cost scenario just because Linux is free," Mastrobattista said.
Giga's warning comes at a time when Linux adoption on mainframes appears to be growing. Approximately 500 mainframe shops have some form of Linux applications running, with an estimated 20 sites running mission-critical applications, according to Giga estimates.