Different channels provide different results--for PPC, a 0.4% CTR is not bad. It's all about getting your offerings out to as many people as possible, and maximizing the channels that work for you and optimizing things you can control.
You're probably going to have better results when emailing your own database and people visiting your website, because they already know who you are (or have sought you out). PPC is a way of getting in front of the people who don't know who you are.
The biggest metrics to look at are less about the CTR and more about the total leads, how many of those meet your qualifications and how many opportunities you get out of them. Work those numbers to get a total cost of leads from PPC and if it doesn't make sense to keep doing it, then reinvest that money in a different channel.