One thing I do to limit bandwidth is to enable DUMMYNET and IPFIREWALL in my kernel and set up ipfw bandwidth caps using pipes. It defies the original purpose of the DUMMYNET module, which was to simulate network configurations, but works fairly well. If you want to look at your monthly traffic as a function of "what constant up and down rate can I get away with and not go over?" you'd need to know how long their definition of a month is and what that equates to in terms of a rate.
Lets assume that a month is 30 days. At 1 Mbps you're looking at about 308 Gigs of traffic. So, to prevent you from exceeding 250 Gigs, your constant rate limit needs to be around 800 kbps up and down.
Given, those are soft estimates, I am unaware of a FreeBSD application that will report your traffic consumed.
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