If it were that easy, this would already exist ¯\_(ツ)_/¯
I think many people use AWS primarily for s3 and EC2 (and s3 costs are negligible for most side projects) so if you were picking a single service, EC2 would probably be the most broadly applicable (and IMO the most realistic one to actually build). On the other hand I would imagine that for many side project developers, their EC2 costs are the easiest to control - even if you do autoscaling, you can cap your max nodes.
What is most useful is tracking the total AWS spend because what really gets you is the service you weren't expecting to cost as much as it did, but that is a very very very hard problem.