We divide our mostly unused basement into two rooms and turned one of them into a small media room.
We had one goal though: managing this on a no-cost budget. So we found old materials, put everything up ourselves and then furnished it with second-hand stuff we found online (except the TV, which already owned).
I think the entire room ended up costing us around Nok 2000,-, which was very acceptable, and we use it frequently to watch a movie together.