Quotes & Sales of Services

We sell services which are tied to a product, on the basis of a price per month. Sometimes on a quote this is an amount for more than one month, e.g. quarterly / annually.

So we would like to have the following columns:

Qty - exists currently - refers to the number of products

Price - exists currently - but we need the ability to specify this as Price / month

No of months (or a more generic term which is obviously not the same as Qty) 

Total - which is Qty x Price / mth x No of mths