Let me answer your questions as much as I can @onemoretime
Cloud Spanner
This is very high level database model in GCP, if you are asking these questions it means you will never need cloud spanner or you are not planning to do something that requires spanner class database. You don’t need it. You can do your own research to understand why it’s too expensive 
My Package (WeivData)
This package is not using external database connection in Wix, it’s designed to switch from wixData codes easily but provided with more features as well as a method called native to support native MongoDB functions.
Library handles some other things on it’s own so you don’t need to deal with them such as:
- Permissions
- Connections to MongoDB Cluster
- Automatic ID Converting
…
And you can customize most of the things. There is also documentation about general questions and a short tutorial of how to setup and use here: https://weiv-data.apps.exweiv.com/
But you will need to write all your database related codes in backend if you choose this package.
Using MongoDB with WixData
You can also connect your existing (or create new one easily with Atlas) MongoDB database to Wix external collections with Wix’s database adapter for external databases.
It’s not documented in Wix’s documentations I don’t know why it exists and works I’ve also test it before multiple times.
This connection would allow you to work with WixData and it’s all features including CMS dashboard visual data binding etc.
You can mix this with WeivData package to achieve things like dynamic pages while accessing to your database with WeivData library.
The main problem you will have (has a workaround that I can’t talk about here) with WeivData is cold start times, which will add from 500ms to 2second to your first database call until backend container is killed.
You can learn more about how Wix’s backend works: https://youtu.be/GCe7aTxyfvA
Each time container is killed your first call to db will require new connection to MongoDB cluster so because of that first action will delayed 500ms-2s depends on your visitor geolocation and MongoDB cluster type.
About MongoDB Pricing
Doesn’t matter you use WixData or WeivData to access your database (or even directly mongodb NPM package) you will use MongoDB.
You can use Atlas which is fully managed paltform directly provided by MongoDB or you can use other external platforms that I don’t have any experience yet so I won’t talk about them but they exists and usually cheaper so you can also check them out in the end it’s both MongoDB database just like Wix’s built in database.
MongoDB Atlas has a free version called M0 Cluster which is probably okay for small workloads, and there is two more cheap cluster: M2 and M5 I guess and after these you have dedicated clusters which starts from 0.08$/hour (around 60$/m).
But I suggest you starting with free M0 cluster and if needed you can upgrade to M10 (lowest dedicated cluster) and then M20 and so on.
Keep in mind that when you upgrade to dedicated cluster you can’t go back to M5, M2 or M0 which are shared clusters.
About other Database Options
Database world is large and there are tons of options, but if you are looking for databases that you can directly work within Wix and something cheap your second option could be GCP Firestore which I also have tutorial about it that explains almost everything about how to setup, connect and work with it. Same tutorial also explains many other details about external databases.
You can read it here: Connect Google Cloud Firestore to Wix (External DB Collection)
If you have any other questions let me know and I will try to answer, especially for the package we’ve built.