I've never seen (healthy) fleets with average CPU utilization above 70-80%. That includes several-thousand machine fleets when I worked at Amazon. Most things I've seen in my career have 10-20% utilization at best. The Lambda "premium" over EC2 is only like 30-50%, so unless your workload is super consistent/predictable, and your code/hardware is well optimized, then Lambda will likely win on price. This is even more true now that you don't have to pay for API Gateway.