CookieClickerPrice is a userscript that shows the amortisation of procucts.
What it doesEdit
This script gives you a suggestion what to buy next as it shows you the amortisation. Amortisation of products is refreshed once you buy something new. It is shown as costs you have to pay for one CpS. The less this is the better is upgrading for a product.
How to useEdit
If you have installed greasemonkey extension for firefox just download the script and restart the game.
Otherwise you might want to read this guide.
(Note: This script might also work on Chrome and Opera.)
You can find the script here: download