There are aspects that will modify this, but I am going to answer just your question.
To host a "city" (I presume that it is one big multiplayer level) it will probably take about 20-30 kb/s per user. For fifty users that will be 1000-1500 kb/s server.
It is usually 10 kb/s up and 10 kb/s down per user, but that will probably rise depending on how much data your game uses.
Also, in the future people, just answer the person's question. Don't post excuses of why he shouldn't do it.