1MBPS = "approximately" 300-320GB/month (each direction)---depending on how your specific host calculates it. I know we consider 1MBPS = 307GB/month. Multiply by the amount of MBPS selected, and you'll have your answer. Keep in mind you would have to saturate the MBPS at every minute of the day to get that usage however, and most people have peaks in their traffic, so do not use that full amount...
I beleive you mean 1Mbs and not 1MBs Capitalization matters. 1Mbs is mega bits per second, MBs is mega-bytes per second.
8Mbs = 1 MBs.
The numbers most commonly quoted are:
10Mbs = 3300 GB/mo in each direction.
100Mbs = 33000 GB/mo in each direction.
Have you considered Semi-dedicated Hosting? 4 - 8 accounts per server. Less overhead than VPS, better performance for the price. DopeDomains.com - 15,000 New Domains a day. indexed by Pagerank, backlinks, Hits, and Length.