Basic tier, 5 DTUs, 2 GB is listed as ~$4.8971/month or $0.0068/hour on this page. Extra storage would cost more but is not available for the basic tier.
Do you have geo-replication turned on? More regions will be an additional $5/month (plus bandwidth between regions) if you replicate. You can serve everything out of a single region but it is pretty easy to add others if you're not paying attention during initial setup.