In a perfect world.
The Holy Grail of home voicover studios is absence, not expense. The absence of extraneous sound.
My cross to bear is the fact that I've got my DAW inside my studio. Computers are notoriously squeaky, whiny, whistly, hummy, and airy-sounding contraptions.
One answer is to get low-noise components: fans, hard-drives, and power supplies. Some places will even BUILD you such a thing. That's probably the lowest-cost workable solution. You can do it yourself. Call Doc Phillips. I seem to remember he's the ace at that option.
Then there are enclosures. My PodWorx friend, Scott Whitney just tweeted me about THIS solution. It's pretty slick. Not cheap…and I have no first-hand testimonials to rely on…but I may have to go with something like this in the end — or just find a way to move the darn box outside the door, like a REAL voiceacting pro.