1668 Adwords clicks, 5 app installs. Is Adwords broken?

Display network conversions are notoriously low. Go to your adwords account and check which websites are giving you very high CTR (15% or more). I am sure you will find a lot of sketchy websites with high CTR. Exclude these websites from your campaign. Since you may have a limited adwords budget I recommend you run highly targeted ads. Finally $100 spent on adwords is too low an amount to reach to any conclusion.

443018 impressions with a click through of 0.37% leading to a 0.29% install conversion rate would be awful be most apps... but you're targeting a tiny niche market - "people with money to buy shares who want a 'free' app to track their portfolio" are going to be incredibly rare. People who bother to track their shares tend to own a lot of them, so a $20 app isn't a problem. Plus, iPhones come with a stock tracking app installed, albeit a bad one, so you've got to get past that too.

I'd hazard a guess that your banner ad is actually very good and is attracting lots of attention, but the product isn't something many people want, hence lots of clicks but few installs. Maybe CPC marketing was a poor choice.

I can't give a profound ad-expert explanation, but when I land on an app, my decision to install it or not depends on the reviews/ratings. Your app has only 2 reviews visible (I can't confirm they are the only reviews) with 4 and 5 stars. Really not bad, but unless a review clearly says it's better than bloomberg, stockwatch and whatever, I would still classify your app as just another one. AND, this is also a big point, human inertia is also a factor, if somebody already use a stock app, the tendency is to stick with what they already know. But you're really on the right track, as you're getting their attention. Maybe you're just missing a deal-maker somewhere, an intangible...