Just checked my server then and it's currently averaging around 10-15kBps per player. Just for comparisons sake back in around January when I was running a Mindcrack Server that was using around 50kBps per player, the current players/maps are no more advanced than they were back then.
But over the past with around 20-60 regular players on at various times throughout the day, I think the most I've used in one month was around 150GB
Bandwidth is cheap in the US/EU so you shouldn't have much of an issue, just use something like dstat or vnstat and keep an eye on what you are actually using.