What do you mean by "recurrent cost"? There is no such thing. I think you mean "recurring cost" - still, that doesn't mean that much though, which leads me to believe that you mean "recurring costs".
"Recurring costs" in project management, are those costs that are happen more than once during the project. Such costs include, but are not limited to, the monthly salaries for employees, the logistics costs directly associated with the project, and 3rd party subscription plans directly associated to the project (such as, for example, a Basecamp account).
Sustaining the recurring costs is simply about ensuring that these recurring costs do not change during the lifetime of the project. For example, having the employees sign a contract that ensures that they can't have a raise during the lifetime of the project will contribute to the sustainability of the recurring costs throughout the project.