Ask HN: Webcam based video engagement tool recommendation

In our start-up, we make edits to the video content (minor edits, like slight changes to the face etc) to make the content more engaging for the audience.

To understand our product market fit, we need to understand if the retention for our videos is better after our processing (we do this for advertisers, so better retention means higher returns for them). I was looking for tools which with a webcam feed can track how "attentive" the person is, but I could not find anything good.

The best resource I found was https://www.realeyesit.com but they are asking us for 17.5k USD (which is obviously unaffordable for a pre-product startup). Some other resources like https://xlabsgaze.com are highly inaccurate.

Do open source tools exist for video engagement estimation using webcam? Alternatively, are there more accessible tools for this purpose (under $100 per month).