In 2012, The Economist reported that mainframes accounted for about a quarter of IBM's revenue and about half of its profits. [0] [1]

IT budgets for large banks and financial firms usually dedicate something like 20% their budget to mainframes, but it usually accounts for about 80% of the firm's workload/processing.

[0]: http://www.economist.com/blogs/schumpeter/2012/09/ibms-mainf... [1]: http://blog.nullspace.io/mainframes.html