These comparisons are fairly meaningless even on IaaS only class deployments. Instance costs aren't necessarily the biggest cost or the best cost optimisation path in the cloud.

For example our S3 costs are higher than our EC2 costs and we made a productive saving implementing lifecycle and S3 IA migration. The biggest EC2 cost reduction we made was through careful analysis and moving some stuff to Lambda, not by changing provider or instance class optimisation.

Whatever happens though, you don't know what the costs are going to be until you get the first bill, regardless of what you estimate.