A formula to calc total number of days elapsed between two dates within any given month

Try looking at this thread

My suggested formula gives you the number of days in each month - you can easily adapt that to multiply by the premium per day

Based on your last post in this thread, its apparent that you are satisfied with the solution(s) you've received and have solved your question, but you haven't marked your thread as "SOLVED". I will do it for you this time.

In future, to mark your thread as Solved, you can do the following -
Select Thread Tools-> Mark thread as Solved.

Incase your issue is not solved, you can undo it as follows -
Select Thread Tools-> Mark thread as Unsolved.

Also, since you are relatively new to the forum, i would like to inform you that you can thank those who have helped you by clicking the small star icon located in the lower left corner of the post which helped you. This adds to the reputation of the person who has taken the time to help you.