This could barely be closer to our exact use-case, so I'm glad you used it as an example - nice to have the decision seconded! We ran for almost a month on the free plan, but were just about to exceed the 'total number of emails sent' limit, so upgraded. MailChimp's pricing automatically adjusts to the appropriate band depending on your current number of subscribers, so although the pricing is convoluted, I don't
really need to worry about it. Even for a small business, £30/month (~ 5,000 subscribers) is very little, and for the peace of mind that comes with a solid offering such as MailChimp, it's well worth it.
I'm sounding like a shill, but I'm really not - just a very happy customer at the moment.