The base app is free to use. For the Mac App Store version there is a 19.99 USD in-app purchase to upgrade to the pro version which includes additional features. In the Mac App Store version API usage is charged based on API requests. 1000 API requests are included with the pro version which should last you a long time. If you exceed this then an additional 1000 API requests are $4.99.

The non-Mac App Store Pro version is 20 USD and lets you configure your own AI API provider such as Open AI or Anthropic.