Prices for plumbers, electricians, or any other tradesman will vary widely depending on area. That said, it seems to me you are struggling with a problem that is, to be blunt, taxing your abilities. In addition, you are messing with property that is not yours, and risking damage not only to your unit but to others as well. My advice to you is to give this to the landlord to decide on how to handle and stay out of it. And, for God's sake, in the future don't use caustic drain cleaners. As you have found, they don't work.