How to score different Ads based on data of sequential steps in a sales funnel?

Question

we run an eCommerce store and constantly create & test new Facebook Ads. Now I am looking for a good way to create a scoring for these ads. Basically it is a very simple problem, but I can't get my head around it.

For each Facebook Ad, I have this data:

• Budget Spent
• Impressions
• Clicks
• Product Page Visited
• Purchases

The most important event obviously is the purchase. So in a perfect world with huge amount of data per ad I would simply calculate the cost per purchase (= Budget Spent / Purchases) and know, which is my best ad. But here comes the problem..

On each ad we don't have much data. So let's say we have:

AD 1

• 50€ Budget Spent
• 10.000 Impressions
• 200 Clicks
• 50 Product Page Visits
• 2 Purchases
• = 25€ per purchase

AD 2

• 50€ Budget Spent
• 10.000 Impressions
• 400 Clicks
• 130 Product Page Visits
• 1 Purchases
• = 50€ per purchase

Simply based on the cost per purchase, I would choose AD1. But when I am looking at the data of the previous steps (clicks and product-page-visits), AD2 looks more promising.

How can I create a score value, that tells me which ad will likely generate the better cost per purchase in the long run, considering also the values of the previous steps?

That score value should take into account the data of the previous steps. So if we have less purchase-data, it should strongly rely on the previous data and if we have much purchase-data, it should rely less on previous data (and more on the actual cost per purchase).

I think of something like:

• Use the click-rate as the base score (because we have much data for this)
• then modify that score with the values of the following steps but in a weighted-way. So the more data we have on the following steps, the more the score moves toward that values.

Thanks in advance for your help!

Best Regards Patrick