[Windows Central Question]

How much does cortana

[saras112]

How much does cortana

It depends on how much you paid for the phone :D

[Pacus1x]

Cortana can work offline for phone related functions like opening apps, notes, contacts and others, those features won't eat mobile data so no money is spend here.

Cortana will cost you money if you want the search feature and if you don't have a mobile plan or unlimited data plan it will burn you data credits.

[ML10]

Cortana is available on all windows phone 8.1 devices. If you mean she costs money to use, it only costs because she uses data like any other app. Otherwise no, you don't need to "buy" her after buying your phone.

[Guytronic]

Does Cortana cost money

Are you meaning does it increase data cost by use?
In that respect yes it can although no more than plain old web searching.
Otherwise Cortana is included with the OS on Microsoft mobile devices.