Hire a Professional
Hiring a professional plumber to replace an old garbage disposal with a new one costs about $300 as of 2010, according to CornerHardware.com. This includes removal of the old unit without needing to change any plumbing or electrical connections for the new one.
Do It Yourself
Replacing a garbage disposal yourself costs about $100, or the cost of the unit. The project takes about four hours to complete. Saving a couple hundred bucks makes sense for those with plumbing and electrical know-how.
If you don't mind getting your hands dirty and have some plumbing experience and the time to do it, put in the disposal yourself.