Algorithms of Google are secret and you will never know the exact relationship between different variables.There are a lot of things that are constantly changing. However the basic elements stay the same.You have to constantly improve you tactics in order to achieve the desired result. We always want to lower our acquisition cost (CPC, Cost/conversion, CPA, CPM etc) however the harsh reality is that all these costs are becoming higher and higher.

• More competitors bid for the same keywords,
• Competitors are improving their websites
• Adwords and Paid Search are becoming more competitive. You will find less broken accounts and campaigns.
• Google wants a bigger piece of the pie and tries it's best to increase the value and the revenues from its properties

Just to give some idea there are some insurance related keywords in the UK that cost 20-30 pounds cost per click. That means that even with a 5% conversion rate they have spent at least 400 pounds for just one customer.  Greece is not like that and cost per click is much lower however CPCs  are becoming more expensive every year.

Google always says that the core components of quality score are the following:

• The historical clickthrough rate (CTR) of the keyword and the matched ad on Google
• Your account history, which is measured by the CTR of all the ads and keywords in your account
• The historical CTR of the display URLs in the ad group
• …......