for free

Without charge, gratis, as in You can't expect the doctor to treat you for free. [Colloquial; c. 1900]

[common] Said of a capability of a programming language or hardware that is available by its design without needing cleverness to implement: “In APL, we get the matrix operations for free.” “And owing to the way revisions are stored in this system, you get revision trees for free.” The term usually refers to a serendipitous feature of doing things a certain way (compare big win), but it may refer to an intentional but secondary feature.

The adverb has one meaning:

Meaning #1: without payment
  Synonyms: gratis, free of charge