Your tech came up with 38GB for a 128k pipe.
The GB <-> kbit/s is a general approximation. The following are industry-accepted values:
monthly avg <-> monthly transfer
64kbit/s <-> 20GB
128kbit/s <-> 40GB
256kbit/s <-> 80GB
(notice a trend yet?)
512kbit/s <-> 160GB
1,024kbit/s or 1Mbit/s <-> 320GB
If you want the exact values, whip out your calculator:
64kbit/s * 60s/m * 60m/h * 24h/d * 30.5d/mo = 168,652,800 kbit/month
168,652,800kbit/month / 8bits/byte = 21,081,600kbytes/month
21,081,600,000 bytes / 1,024bytes/kbyte / 1,024kbytes/mbyte / 1,024mbytes/gbyte = 19.6337GB/month
This is assuming 30.5 days in a month. If you want to be precise, calculate the number of seconds in the month in question and rework the problem with that. That's going to be the most precise approximation you can get (precise approximation -- silly huh...).
You'll get different "exact" numbers from different places because of different days-per-month values and the current position of the moon (that affects everything you know). However, everyone will come out with an approximation of 64kbit/s <-> 20GB. To make life easier on me, I just use this formula:
64kbit/s / 3.2 = 20GB. I divide kbit/s by 3.2 and get my GB. I multiply my GB by 3.2 and get my kbit/s. It's not exact, but it's close enough for my application.
"Close enough" only counts in horseshoes and hand grenades. I make it count towards monthly bandwidth calculations, too.
If I made any errors anywhere, please correct me. I blame the calculator on false information above, my fault or not.