The trap
Broaden targeting, drop into cheaper placements, chase low-competition terms — CPC falls and the report improves. Conversion rate falls faster, so cost per order rises.
What to watch instead
Cost per order against your maximum allowable CPA. That is the only cost number that connects to the business.
Where cheap clicks are genuinely good
When they come from better relevance rather than worse traffic. A more relevant ad and landing page lowers what you pay for the same position — that is the honest version, and it shows up as CPC down and conversion rate steady.