If you search for something along the lines of "paint and sand 3d print" or "finish 3d print" you'll find lots of different articles.

Edit: $59 mentioned lower in the thread.

For a prototype which is expected to drive some interest 3d printing is fine, but print at 0.15 layers and do a couple of runs of sanding combined with filler before applying some paint. The case will look just like a regular finished product. I do this for custom projects where I want a perfect finish and/or show-off prototypes.

The layering is really annoying for some people.

For a final product, depending on the sale numbers, injection molding might be the only option. SLS and MJF give a very nice result, but the price range where they're economically viable is very narrow.