I've paid $0.00 for my many AWS Lambda functions, and will always pay $0 so long as I keep it under 1 million executions per month (and AWS keeps this free tier going). I also use SimpleDB, SES, and other services that also cost $0.00/mo. The only thing I pay for is S3, which totals about $0.30 per month, for a few gigs of data. And I'm very happy with AWS, It's been costing me less than $6 per year to build out and run a complex web app, albeit with no users. But still, AWS absolutely does scale down to zero very nicely.