At the beginning of the month I switched the hosting of my copy or River4.js from Heroku to CloudAtCost. The move was initiated by the fact that I needed to update the my instance of River4 and I couldn't find a way to do it given how I had originally set it up on Heroku.
It was pointed out to me that there would likely be a cost impact of my hosting River4.js on CloudAtCost and using Amazon S3 for file storage. Because Heroku is hosted on Amazon you don't get charged for data transfer because the data traverses Amazon's internal network. I was skeptical, but after running on CloudAtCost for a month and monitoring the bill for S3 I can confirm the higher cost.
After nearly a month, there has been a little over 58 GB of data transferred out of my S3 bucket for a cost of $6.99. Last month I only had 1.77 GB transferred out costing me a whopping $0.14.
So, from a financial stand point, it seems to make sense to host River4.js on Heroku as opposed to on CloudAtCost. I think I'll make the flip and see how I am charged on Amazon in October.