Hi ya Jennifer,
To see if your ROI (return on investment) was worth it with facebook ads (or with anything you pay and are looking for an outcome with). You need to decide what you want the people who click your ads to do. If you did the ads for people to just click and visit your site then from $20 100 people clicked that means for every $5 you spent your ROI was 25 click throughs (so for each click it costs you 0.20c).
Now if your objective from your ads was sales. First you should know how many sales came from your ad click throughs. Then work out the ROI on that. If your objective was to get sales, and no one brought from you then your ROI was $0.
If 10 people brought via your ad then each purchase was $10 that means you made $100 from your ad campaign.
So $20 (your investment) / 10 (people brought) = $2. This mean for $2 for ROI was £100. Only you can decide if that was a successful campaign or not. An from your results and elevation you would need to look at your ad and the information it included, was it aim at your ideal customer etc…
A great resource: http://www.socialmediaexaminer.com/measure-social-roi-on-facebook-ads/
I hope this helps
Blessings Janet