Junction box installation costs $100 to $300 for parts and labor, depending on the installation location, accessibility, and the electrical box size, material, and indoor or outdoor rating. Plastic junction boxes for indoor wiring cost 50% to 80% less than metal boxes but aren't as durable. The final price depends not only on the box itself, but also on. The cost of replacing a junction box will depend on a variety of factors, including the age of the current junction box, the size of the new box, and the wiring and labor needed to install it. Here's a simple, user-friendly guide to help you through the process. A precise cost estimate helps homeowners budget accurately for safety upgrades and code-compliant electrical work. Cost ranges reflect a typical.