The available size of an AuraDB Instance and its cost is displayed when you click on the Create Instance option in the console page.
You will see the different sizes and resources available along its cost details if you select the Aura DB Professional Tier:
The updated pricing details can be found here: https://neo4j.com/cloud/aura/pricing/
An AuraDB Instance priced at $0.18/hours will cost you 24 hours x $0.18/hours * 30 days = $129.60 per month.
Please refer below link for Billing FAQs: https://aura.support.neo4j.com/hc/en-us/articles/360037061854-Neo4j-Aura-Billing-FAQ