That is the best part! It is cheaper! We had a node service that went from $2500/mo to $400/mo just by switching form Ec2 to Lambda. That is the most apples to apples comparison I can give you. Other services were split up or rewritten, but honestly my gut feeling tells me they are costing 10x-50x less.
IMO Autoscaling ec2 instances to daily traffic is not a trivial problem and you'll never get the granularity you can with Lambda
You are also saving in not having someone maintaining and making sure the server is always online. Even if it was the same price, not having to worry about the server is a big plus
IMO Autoscaling ec2 instances to daily traffic is not a trivial problem and you'll never get the granularity you can with Lambda