Start with a no-brainer price that's easily justifiable based on your current credibility. I believe 'lifetime' is a little bit of trickery because it doesn't state the lifetime of what? Your interest in the project? Your human life? The lifetime of the technology?

It's useful for a money-grab on your part so set the expectations accordingly.

I bought somebody's entire productive future FOREVER for $1,000 -- 3-years later, it's still going. So, good deal.

( You can always increase this price at any time. )

I'd say that the 'interest' life in a tool like this maxes out at 3-5 years. So, if you could get lifetime for 3x the annual rate, it feels there's a shared risk.

Estimate a lifetime value. Something like a)user will use the service for X number of months and than cancel. b) if i know that the user will be around for x number of months, what would that normally cost c) maybe offer a discount and (d)work out how much you will reinvest . (have not done this but I understand that if you subtract card processing fees, you can offer a good discount and still make something on the deal).