Well, look in your UBB control panel .. you should see how many hits you have gotten so far this month, now .. do the following (this will only give you a rough guideline) ..
1. Divide the number of hits by the date of the month. (i.e if you got 2500 hits and it was the 15th of the month, your answer would be 100.)
2. Multiply that answer by 40. (i.e, 100 * 40 = 4,000)
3. Multiply that by 30 (4,000 * 30 = 120,000)
And, the answer you get, is how many kilobytes of bandwidth your UBB takes up per month based on current visitors. (Please note, in step two, it is based on your pages being roughyl 40Kb large, if your pages are all 500Kb large replace 40 with 500.
So, just to go over it again, this time in algerbraic form:
x = Hits / Date of Month
y = Rough averge size of your pages (in kilobytes)
(x * 30) * y = Rough bandwidth usage per month on Kilobytes.