1.Invest in reserved instance and Spot instance also good option to optimise cost.

2.Rightsizing- Choose the correct resource size for you application and your server.

3.Remove unused and unattached snapshot/resources.

4. Minimising data transfer cost using same region- make sure your object storage and compute Services in the same region because Data transfer is free in the same region.

5. Use autoscaling to reduce cost during off hours.

6. Also, we can use cost optimisation tool like cloud checker etc.

Cost optimization is not only on compute level. It can be achieved with right design as well.

As example one company will have multiple direct  connect to AWS for each account and can be bearing lot of Egress traffic and other company may be using shared vpc model which will save lot of egress traffic cost. So its always crucial to have correct high level design which is cost optimized before even thinking of compute migration.