Now when you buy a new Mac at an Apple Store they will migrate your data over for free. Previously this service cost US$99 (via TidBITS).
Data migration was originally part of Apple’s One to One program. This was a training program offered at Apple retail stores. Through this program that cost US$99/year you could learn more about your Mac and get assistance from an Apple retail employee. The program was shut down in 2015.
But now when you buy a new Mac in an Apple Store, an employee will migrate your data from your old computer to your new one for free. For more knowledgeable users, macOS has a built-in Migration Assistant where you can do this yourself.