Can I set up different databases for different client under my own Azure account? (these will all be very small databases)
You can configure permissions to database to specific LOGIN by creating USER in that database.
You can configure separate virtual server for each client (virtual server do not cost)
You can control access to specific database or the virtual server for specific IP
Can I set up multiple subscriptions, 1 for each client,
Yes, you can open as many subscription as you want
or is it better to have them all under 1 subscription.
This is not a yes/no question since the answer depends on your specific scenario and what your personal preferences.
Using one subscription to all means simpler control but less flexible and mistakes can lead to leak of information between clients (for example you might create a database in the wrong virtual server, or give permissions to the wrong client on someone else entity.
When I pass on the cost to the client, which will be about $8 per month, can I realistically add on a bit to make it my worthwhile and for maintaining it?
Question is not clear to me. What
$8 means? What do you mean by
can I realistically add on a bit to make it my worthwhile?!? If you can sell ice to Eskimos in Antarctica then his is your business and you are welcome to do it.
This forum is not for marketing discussion but for technical discussion. This is something you better discuss with marketing people.
What you should know is that:
The cost is the same in theory if you have all in one subscription or multiple, since you pay per service and not important if the services are in the same subscription or not