The expanded formula is the following:
Costs MTD (Cost of Instance Month to date) / hours (running in the current month)
It's based on what costs are recorded against the instance in the current month divided by the hours it has been running in the current month.
The Costs include EC2 costs (including Reserved / OnDemand Usage) plus any EBS costs so there is no set rate that can be determined from this hourly rate as the cost depends on the various Usage types and EBS costs.
If you want a break down in more detail, the best method is to use the instance hours report here:
You can edit the columns in this report and add the following:
Amazon EC2 Instance > Hourly Cost
Amazon EC2 Instance Execution > Total Cost (combines EC2 and EBS costs)
Plus any other columns you find useful.
The Total Cost / Total hours should equal your Hourly Cost.