We use AWS Lambda in a couple of our projects. In one project, we could have solely used Lambda and API Gateway if API Gateway could be protected inside a VPC.
Anyway, AWS Lambda based services of our system cost us less than $5 per month on QA and Prod environments combined. In comparison, EC2-based components of our app cost us a few hundred dollars per months per environment.
I know this post is about not getting locked-in to a certain vendor and there are some FOSS options that allow setting up FaaS/serverless environments both in private data centers as well as in the cloud. One is an Apache project lead by IBM called OpenWhisk: http://openwhisk.org/
The other, still in its infancy IMHO, is called Fission: http://fission.io/
If you have enough functions and a stable load that can keep your servers busy all the time or if you don’t care for the extra cost incurred for when your servers are idle, these are some viable (but still maturing) options to AWS Lambda.