"20-30 kb/s per user"

Reduce that by about 10. If you are pumping that much per user, you are sending too much and/or being too sloppy with your network code. Around 2kb/s per user should be your target bandwidth.

This means that you'll be able to host a big city on a home cable modem line though it might not be stable. A dedicated line runs about 300 USD per month (plus installation) at 1.5 MB/s up/down.

Test at home with a cable modem and then move up to more expensive solutions as the need arises.