Calculating Contract Value

I am writing a business plan for a new ventrue that our company is considering and I need to figure out how to calculate remaining contract value.

Imaging that a company sells 10 contracts per month and each customer agrees to pay $50 per month for 60 months. In the first month the total value of all contracts would be $30,000 (10 people, 60 months, $50 per month). The second month, the total value of all contracts (assuming 10 new people and 10 people from last month) would be $59,500 (10 new people, 60 months, $50 per month PLUS 10 existing customers, 59 months, $50 per month).

What I need to do is to create a running total value of all of the remaining contract payments out to 60 months - taking into account that as months go by old contracts will age by a month and will therefore go down in total contract value.

Is there a way to do this without having to do it by hand in each cell of each month?