I've pushed teams to choose Spanner over using Postgres (when already in the GCP ecosystem). It's not really more expensive when you sit down and do the math, you save untold hours of maintenance over the lifespan of the app, and it actually scales without fuss.
what kind of maintenance?
I think one of the maybe less talked about benefits of distributed SQL is support for nearly transparent rolling upgrades of the database with very little impact to a running workload. Spanner is best in class at this.
Is it really not more expensive? I haven't operated a large scale Spanner cluster, but the numbers would suggest it's quite a bit more expensive. I suppose it's workload dependent is what you're saying?