I ran some rough numbers a while ago and I found single-region GCP Spanner to be the most cost-effective relational DB option (out of the ones I looked it) if you don't mind that it can't scale to zero (min $65/mo), if synchronous replication for durability is a must and you need serializability. It's more cost effective than any Aurora offering including DSQL (which isn't serializable afaik but even then).