IMO the most important advantages come from:
1. Having your own room, so you can do/listen to what you want, regardless what the room is like
2. The most important aspects deal with design (size, shape, features) many of which are cheap/free if you building anyway.
3. Conversely mistakes in design can be avoided with a little knowledge.
I got a man cave when we built 4 years ago, so I followed Cardas recommendations for room dimensions (free). The interior walls adjoined storage spaces, so isolation was already provided. The builder insulated all interior walls for general soundproofing, so that was taken care of. I specified lined/insulated flexible ductwork to the room which was actually cheaper than typical metal duct. I also specified an insulated fiberglass exterior door (with threshold/weather seals) that was cheaper than the other interior doors in the house. I got three dedicated circuits for the audio system (no cost as the electrician bid the job at a flat rate). My splurge was buying three cryo'd hospital grade 20 amp receptacles, but the net additional cost was very near zero for the room.
My mistakes were in the ceiling (below kitchen/laundry/wooden floors). I specified six recessed can style light fixtures. Next time I'd go with wall sconce light fixtures if built in light is needed. And the builder wouldn't "float" the ceiling on gauge metal furring strips as the drawings called for. So as a result of both, I hear everything above and they hear my music from the kitchen/laundry. This could get fixed with green glue, a 2nd layer of drywall, wall sconces, labor, mess, temporary loss of the room, and of course money.
Bottom line, except for vertical isolation, the room works great and cost next to nothing. I have privacy, my own space, and a near ideal listening environment. As a bonus wifey has less space for "nest building" (fill with "junk" that she spends money on). A win/win situation.
