Our servers are costing us around 600 usd per month, Linode and AWS. Here is a screenshot of Linode manager and AWS billing console: http://imgur.com/a/UUQ65. The names of the servers perhaps give it away, but we use redis for message passing, 4 servers that crawl around 100 000 feeds every 10 minutes, 2 cassandra databases storing posts, 1 mega MySQL database for metadata and users (post lists, unread post lists, etc). AWS for SES (e-mail notifications) and S3 (extremely cheap key value store for large items, post contents).
Edit: Oh, and after a really bad downtime we're paying for pingdom and statuspage.io. So our total expenses are now probably around 1000 USD per month. Braintree and PayPal also take around 5%-19% of each payment.
Then we have additional costs of http://chartmogul.com (love. required to know your SaaS metrics) http://intercom.io/ (love love love, awesome product).
Edit: Oh, and after a really bad downtime we're paying for pingdom and statuspage.io. So our total expenses are now probably around 1000 USD per month. Braintree and PayPal also take around 5%-19% of each payment.