Search Soundproofing a door and your bound to find a myriad of confusing advice including using styraoam panels, egg cartons and who knows what else.
The 3 main components to properly soundproofing a door is;
- Mass
- Damping
- Sealing
Get those three components straight and you’ll have a properly soundproofed door for a fraction of the price. Here is a straightforward article explaining these three steps
http://www.tmsoundproofing.com/How-To-Soundproof-a-Door.html
If you prefer to buy a ready made acoustic door be prepared to spend between 2 to 4 thousand dollars something like this one http://www.kriegerproducts.com/acoustical/