How is the billing in the Aura console laid out ?

The Aura console provides a great level of detail for the user and allows you to measure the costs on an interactive basis 

• The billing presents a breakdown of the costs per AuraDB Instance (see #1) and the number of hours is listed (see #2) to clearly allow you to verify and understand the costs. 
• In case of discounts or credits (See #3) that are applied against your usage (to correct previous billing or a service issue) you can see them instantly when a coupon is applied against your account. 
• The total due is updated with the latest usage figures and can be consulted live (see #4) and it can present a credit or debit total amount.  
• You will potentially see multiple Instance States if the instance has been paused during the billing period, it will be paired with the usage and a total amount, this is the total billed for the instance while it was in that state, this is not the current state of the instance.

Finally, please note that as highlighted here, so long as an AuraDB Instance is running (used or not) or paused, it will incur costs.

Unit price (Per GB-hour) 

You will be able to see a breakdown of 'Unit price (Per GB-hour) and Usage (GB-hour(s)) in the Aura Console under the 'Current Month Usage' section.

The actual hourly cost is calculated by multiplying the Per GB-hour with the Memory size of the instance.
So, if an 8GB memory instance is run for one hour, you would have used 8GB hours.

At the time of writing this article (May 2023), the Unit price (Per GB-hour) of a running AuraDB instance is $0.09, and that of a running AuraDS instance is $0.125

With a Unit price (Per GB-hour) of $0.09 and 8 GB usage hours, a running 8GB (Memory) AuraDB instance would cost $0.72 per hour (8 x $0.09 )
With a Unit price (Per GB-hour) of $0.125 and 8 GB usage hours, an 8GB (Memory) AuraDB instance would cost $1.00 per hour (8 x $0.125 )

To keep things simple, you can use the hourly price shown during instance creation as a point of reference.