Those aren't cheap, but rolling your own usually isn't any cheaper. Even huge enterprises usually buy instead of build because it's cheaper in both the short and long run.
Think about most managed cloud services - you could deploy your own SQL servers on EC2, configure replication, fail-over, backups, security patching, log collection, observability, etc. - but you'll end up paying a lot for engineers to build, maintain and monitor that solution compared to just spinning up one of the ready made offerings by AWS.
It might be cheaper to do if you have a ton of RDS, but it really has to be a huge huge volume, and even then, AWS will probably find a way to discount your bills to make it still better...
Think about most managed cloud services - you could deploy your own SQL servers on EC2, configure replication, fail-over, backups, security patching, log collection, observability, etc. - but you'll end up paying a lot for engineers to build, maintain and monitor that solution compared to just spinning up one of the ready made offerings by AWS. It might be cheaper to do if you have a ton of RDS, but it really has to be a huge huge volume, and even then, AWS will probably find a way to discount your bills to make it still better...