It isn't for doing security screening, there's no way $5 would be worth the time of someone professional enough to look at your code for 20 or 30 minutes, God forbid if it's longer than that.
There is a way to make app review "for safety" scale at this price point: be ultra restrictive about what apps are allowed to do, and review them not as much for security as for compliance to API guidelines.
•Cough•.
In this case, this is a browser extension, and the goal is to edit the page: there are tons of such extensions; while a couple of these had obvious "talks to the wrong server" problems, various of them were correctly only editing pages on Facebook, but were doing so maliciously.
The price point we are talking about here is simply so low: $5 is only going to purchase 40 minutes of a minimum wage employee... we aren't even talking a junior supervisor at a fast food restaurant, we are talking about the entry-level "try to get the orders in the computer right" position.
At these prices, even just reading the description and figuring out "oh, this should only be able to edit pages on this one website" is already going to be expensive. Figuring out "should only be using DOM to remove nodes and not add script elements" is impossible.
Chrome does this on some level via host permissions (http://developer.chrome.com/extensions/manifest.html#permiss...) and a content security policy (http://developer.chrome.com/extensions/contentSecurityPolicy...) and is setting a more strict default security as manifest v2 gets phased in.
edit: On perusal, I see it's not even $5 per add-on or publish, it's a one-time fee to register as a developer.
The unnecessary cost and additional process (e.g. requiring screenshots) are all reasons LikeBuster (https://addons.mozilla.org/en-US/firefox/addon/likebuster-fo...) is available on Mozilla's Firefox add-on site but needs to be manually installed (http://github.com/relwell/LikeBuster) for Chrome. Knowing that the extra b.s. doesn't actually accomplish anything for user security really grinds my gears as someone who would like to make it as easy as possible for people to download my extension.