http://myopendraft.blogspot.com/2009/09/6060-rule.html
Fully 60% of the life cycle costs of software systems come from maintenance,
with a relatively measly 40% coming from development. That is an average, of
course.
The actual cost of maintenance may vary from 40% to 80%, depending
on the system type and the environment it is deployed into. During maintenance, 60% of the costs on average relate to user-generated enhancements (changing requirements), 23% to migration activities, and 17% to bug fixes. The 60% of life cycle costs related to maintenance, coupled with the fact that 60% of maintenance activities relate to enhancements, gives us the so-called 60/60 Rule, one of the few proposed “laws” of software maintenance.
