Monthly: $20 per container + CDN usage.
A container would be the staging environment, or a production environment. For most of your projects, it would just be one container for production ($20 /month + CDN). For actively developed projects, it would be production and staging containers ($40 /month + CDN).
CDN costs are in the pennies for slow sites, and upwards to several dollars for active sites. Pricing is set by Google, and we just pass it along to you. https://cloud.google.com/storage/pricing