I think the subscription/"free trial" concept is a particularly hard sell for this. Maybe micropayments would be much better suited as a default: pay $1 right now via Paypal to have a video transcribed. Subscription optional for recurring customers.

I’d go as far as to say it’s very useful :-) I don’t know how many times I’ve sat trying to skip trough a video to find a certain phrase or fact. Also the name is great and easy to remember, definitely committing that to my memory.

I am curious, from a technical point of view, are you pulling down the YouTube transcription or doing your own? And does that rely on the YouTube api (thus costing money for you at some point?).