You aren't the first to suggest this and it would possibly make it easier for people to get started. There are two main problems though: monetization (as you brought up), and also the fact that many privacy-conscious individuals may not even have a Dropbox account to begin with because of its implications.
I'm always open to suggestions though. If there was a viable way to charge for Turtl while also making it easier for people to adopt, I'm all ears =]. I think it would be hard to charge a one-time download fee for an open-source app though (and open source is essential for anything dealing with crypto).
I'm honestly not sure what the best way to do that would be. Perhaps a guide on your website of how to configure the server to sync via Dropbox (or other syncing folders), but with a request for donations at the top and bottom of the article. "If you found this useful, please consider..."
I'm always open to suggestions though. If there was a viable way to charge for Turtl while also making it easier for people to adopt, I'm all ears =]. I think it would be hard to charge a one-time download fee for an open-source app though (and open source is essential for anything dealing with crypto).