Most of the listed sources will do what is known as "dip only". You send them the fully disassembled gun and all they do is run it through the cleaning and bluing cycle.
They charge less for that since they do no polishing.
As a final possibility is Ron Mahovski. He does cheaper bluing, but you have to wait a while, delivery likely won't be when promised and communication is poor.
His specialty is plating, but he "fires up the tanks" once a month or so.
http://www.mahovskysmetalife.com/