Your monthly estimated billing is fully based on your hourly billing multiplied by 24 * 30 = 720
Now this is only a estimated amount and the actual price may differ with a few cents due to different length of the months, leap years etc. For a more accurate calculation you could apply this to your hourly price 24 * 365.25 / 12 to get the average monthly price based on the accurate calculated yearly price.
Should your price against all odds be wrong you may contact support to have it adjusted manually.