Cost of replace valve behind toilet and replacing the complete kit inside the toilet. (flapper, floater, stem)

A 1/4 turn ball valve about $6; a new line to toilet $4; a toilet kit $20; all together about $30 plus tax if you do it yourself. Speedball would know how much a plumber would charge for the job.

You're looking at about a hours labor to do the installation. Not knowing what service calls run in you area makes it difficult to place a labor rate.
Good luck, Tom