I have column B with pricing data and column C stating the billing plan.
The billing plans are either monthly, quarterly, or yearly.

What equations should I use to sum these based on the billing plan?

For the billing plan column, should I use numbers instead of words? (12 for yearly, 4 for quarterly, 1 for month)

Example:

A B C
January 2016 Price Plan
$10 Monthly
$40 Quarterly
$120 Yearly
February 2016
$10 Monthly

January 2016 Sum =$170
February 2016 Sum =$20
March 2016 Sum =$20
April 2016 Sum =$20
May 2016 Sum =$60
...
January 2017 Sum =$180