Your calculations are a bit off, I'm afraid. But you're smart to start looking at the issues.
500 subscribers at 150 mails per day is 75,000 mails. 75000 x 1K is 75,000K, or 73.24MB or 0.07GB/day. In 30 days, you would need 2.15GB of bandwidth.
If you're allowed to install MailMan on a host, make sure it has TONS of memory. Our MailMan installation runs very, very high on memory and it has been told that this is a "feature" by the MM dev team.
If you're not up to doing your own installation, find a provider that already sells lists powered by MailMan.
We do it but there are others out there, as well.
Nathan