[giladvdn]

MailChimp allows you to buy credits as you go, or pay them monthly by subscriber count. It's also built for marketers so after importing your list there your marketing guy should be fine.

If you're looking to do it cheaper, SendGrid offers a newsletter function that's less easy to use and has less features but will allow you to do it cheaper.

[twog]

Mailchimp is really, really expensive for anything this size. I contacted SendGrid & so far they look like the front runners