We're currently implementing monthly subscriptions as a payment method via Paypal and Coinbase (bitcoin payments). Our two most common payment methods at the moment. The only trouble with this is the lack of ability to easily calculate varying charges such as temporary servers, additional features or bandwidth overage. But in case you're setting up a long term server or set of servers that you know will have a constant price every month this may be a useful option.
For bandwidth overage (on affected plans) I think a proper solution would be the ability to decide if a server should turn off to save money or keep running (with additional fees). For that we need a settings page in dashboard which should be available in a near future.
Last but not least we may add the ability to create a budget that includes customized notifications over email like most other cloud providers has. Notifications that compare the current months usage with one or more user defined "budget alert" values. This may replace the current notifications completely as they're inaccurate in case of temporary additional fees due to live calculations.