When you are on the 'Budget & Schedule' section of your Ad Set, set your budget to $1.00.
Scroll down to the 'Optimization & Delivery' section.
Click on 'More Options' next to 'When You Get Charged'.
Change when you get charged from 'Impressions' to 'Link Clicks (CPC)'.
You can now run your ad for as little as $1/day!