I've been attempting to monetise my Google Chrome Extension for the last few months ( https://news.ycombinator.com/item?id=12925467 ). I felt I could never inject ads to my extension as I never had a sensible place to put them.

Instead I've been experimenting a lot with one off & subscription approaches, here are two things I noticed:

1. When I had a subscription model, most users would sign up & almost instantly cancel their subscription (though very few ever requested a refund).

2. Adjusting the one off price didn't change the monthly revenue. For example when I changed the price from $7.99 to $10, the sales decreased a little but at the end of the month I ended up earning about the same.

(Shameless plug: https://chrome.google.com/webstore/detail/livepage/pilnojpmd... )