For most things I personally use AWS Lambda and S3, with some SimpleDB. If I needed something more in the DB department, I'd probably use DynamoDB, but S3 actually works well for most of the data I need to store. I pay about $0.50/mo and most of that is S3 cost for media files. I don't have a lot of users, but even if I did I'd still be paying way less than $20/mo. And when I do have a lot of users, I don't have to worry about scaling anything (and that will happen soon). I think the first million Lambda invocations are free, and if/when I start paying for Lambda invocations, the AWS costs will definitely be covered easily by user subscriptions.
I also use pretty much the same stack at work, where we currently have about 150,000 users. The cost is about $15/day, mostly in S3 costs, we have a lot more data at my day job. We also do use a few hundred EC2 instances in short bursts, and the VPC has stupid high costs, and other costs add up, but it's probably about 1% of what the rest of the teams cost that run on EC2 and Postgres and other AWS tech.
I also use pretty much the same stack at work, where we currently have about 150,000 users. The cost is about $15/day, mostly in S3 costs, we have a lot more data at my day job. We also do use a few hundred EC2 instances in short bursts, and the VPC has stupid high costs, and other costs add up, but it's probably about 1% of what the rest of the teams cost that run on EC2 and Postgres and other AWS tech.