I would also add that the message (text) of your ad has a major influence on your conversion rates.
For example, I was just looking at the results of a test I was running on callout extensions and noted adding a 2-word phrase to the exact same ad had boosted conversion rate to 27.45%, a 97% lift from the ad variants without that phrase. This same phrase has shown a similar boost in rates across a wide range of keywords and products within this particular niche.
We haven’t even added this phrase to the landing pages yet, that will be our next set of content experiments. I will not be surprised if we see an additional boost from implementing this marketing insight into the landing page content, as well as the checkout process.
The bottom line is that your particular conversion rate isn’t likely to be anywhere near another website’s conversion rate, except by pure random chance. There are just so many variables to consider, for which you do not possess any data until after you have launched your campaigns.