As you know Azure Cosmos DB will cost high if you are spending time on learning & experimenting. Alternatively, following are different options inside/outside the Azure Portal to try with:
MSDN $200 Free Plan
https://azure.microsoft.com/en-us/free/free-account-faq/
Emulator
https://aka.ms/cosmosdb-emulator
Sandbox
https://docs.microsoft.com/en-us/learn/modules/create-cosmos-db-for-scale/2-create-an-account
