what would you do to keep server bandwidth low as possible? And what would be your projected server bandwidth, for only movement?
First, why are you so worried about low server bandwidth cost? Once your game is of any reasonable size, you'll be paying $2 per megabit per second per month inside some co-location facility if you want cheap bandwidth, or $8 per megabit per second if you want fancier bandwidth. That's something like $0.0008 per gigabyte of data.
Second, the actual numbers: If this is a RPG, you'd do interest management, where perhaps 10 players and 10 mobs would be of high interest, and sent perhaps 2 times per second, and 90 mobs and 90 players would be of low interest, and sent perhaps once per 5 seconds. (This is based on WoW-style RPG play mechanics.) Let's say that id + position + velocity + target + facing is about 12 bytes per entity. So, this is 10*12*2+90*12*1/5 == 792 bytes per second per player. 79 kB/s for 100 players. Not something I'd worry about on the server side.
Let's consider a MMOFPS, where you'd need your high-interest entities at 20 pps and your low-interest at 5 pps; you'd get 10*12*20+90*12*5 or about 7.8 kB/s. 100 players make that 780 kB/s -- you're at about ten megabits per second at that point.
Now, count packet overhead. In P2P, each player sends data to each other player, so each player sends at least 198 packets per second in the P2P case, and receives 198 packets, for RPG, and sends/receives 1998 packets per second in the FPS case. This is a rather high packet rate, that many residential firewalls/routers won't even be able to sustain. Then each packet has overhead -- let's count 32 bytes for framing and IP/UDP headers; that's 64 kB extra in packet overhead per player in P2P. That's almost as much as the entire payload data stream for the server to send data to all players!
Even a super-expensive bandwidth provider like Amazon EC2 charges less than $0.10 per GB per month (which translates to about four hours of payload for the hypothetical worst-case FPS gameplay session above.)
Peer-to-peer: Okay for distributed bulk file sharing; usually not the right answer for game networking.