I just got my Amazon Web Services bill. It looks like the portion of it attributable to running River4 for August was a little over ten dollars. I know this is not backbreaking, but it's not free either. It's more expensive than a paid account at Newsblur or Feedly. And I'm already running Fever on my VPS for no extra cost. Right now I'm liking my river but the cost of the Amazon bandwidth and service requests are higher than I had anticipated.
I remember going through a similar experience when I was running River2 and River3. At that time, the big cost was running the EC2 instance 24/7. With River4, I'm running the software on my VPS, so it's not costing me anything extra. It's all Amazon.