An in-home theater can cost as little as a few hundred dollars to build, or it can cost tens of thousands. It all depends on how elaborate you want your theater to be.
If you're just looking for a basic setup, you can get away with a small projector, a screen, and some surround sound speakers. This will set you back a few hundred dollars at most.
If you want something a little more luxurious, you can add things like reclining chairs, a popcorn machine, and a fancy sound system. This can easily cost a few thousand dollars.
And if you really want to go all out, you can build a dedicated home theater room complete with a big screen, a top-of-the-line projector, and a surround sound system that will rival any movie theater. This will cost you tens of thousands of dollars, but it will be worth it for the ultimate movie-watching experience.