I have read that Google considers the link from your Adwords target url page in calculating qs. A lot of the times this is an affiliate offer page. Does anyone have any data backing this? Can this page be simply a call to action to get a free download or does Google want to see something of monetary value for sale?
One major factor in the QS is relevancy. Your keywords and ads must be relevant to your landing page. Choosing "dog food" and having your ad say you are selling dog food is relevant. But if your page is trying to sell cat food, it is not relevant and you will take a hit in the QS and your cost will be higher. If your page is simply one where you ask for an email in exchange for a free download, your QS will suffer too. You need to have a bit more than that. If you want to know more, especially about quality score, download my Adwords FAQ.