Reoccurring monthly contract payments, with newer contracts added monthly

Hello,
I have a facility that is membership based and grows each month when new clients join and start new memberships. I want to figure out how much total revenue a sales clerk generates by selling memberships.

1. Memberships are 1 year long
2. Billed once a month
3. Price is $59

Here is my representation of what I want to accomplish, I just need an automated formula.

1. Employee1 averages 25 sales/memberships a month
2. Question: how much revenue does Employee1 generate on at the end of each month, at the end of the year? It's not linear, it's aggregate.

Below is the physical representation I just don't have an automated formula I had to make this chart to get the monthly and year totals.

Screen Shot:
Excel Memberhsip Rev Chart.GIF

Here is a 2nd direct link of a screen shot:
http://i97.photobucket.com/albums/l2...ps3569e48e.gif

Thank you so much :-)