You need to know this, and knowing it is as easy as asking.
I e-mail every single person that cancels their subscription, and ask why they cancelled today and what we could have done to better serve them. I don't measure the response rate, but I write every mail personally and read every response, so I can tell you it's well over 50%.
The responses to these e-mails are invaluable. They're how you learn why people signed up, what pain points they experienced using your product, what problems they have that your product didn't solve well, and what competitors they're coming from or leaving to.
If there's some glaring issue with your ad campaign ("I thought I was signing up for a free antivirus program because your ad said it made my files secure" or "No hablo ingles") you'd probably have learned it well before spending €20K through those exit emails.