Here's an overview of 3D printed molds for injection molding.[1] These aren't the first people to do this. The mold has to have a much higher melting point than the material being molded, of course.
Formlabs has a "high temp resin", melting point around 289C.[2] US$200/liter. This is for their stereolithography machine. Filament-based 3D printers, not going to work for this.
This makes sense for production shops that need to make a few hundred of something.