The State is free but TT charges $19.99 to e-file the return. If you choose to e-file both returns, you pay $19.99 per return. You could avoid the charge by printing and mailing the return, but e-filling is faster and easier, but not free.
If you were paying for a state, say your son lives in AZ, and you in CA, before you started his AZ return, TT would ask you to pay for the download.
