Below are couple of best practices which will enable to keep your Azure SQL cost low
1. Make sure you select the right price tier. I have seen that many admins select Premium however they don’t ever use required DTUs.
2. Make sure you have retention planned on your Azure SQL DB. If this is not done then you need more resources and hence you need a higher tier to manage your Azure DB.
3. Push the data to blob instead of using Azure SQL in case the data is only for the retention purpose. The price of the blob storage is dependent on redundance level and accessibility level.
4. I always encourage my customers to use Azure price calculator to estimate the Monthly Azure bill – https://azure.microsoft.com/en-us/pricing/calculator/
5. Use elastic pool wherever necessary to manage multiple DBs in one flexible pool