Well, I was specifically replying to infrastructure costs. But I can address non-infrastructure costs too.

Most important is that there is no need at all to make an account first. You can stick a "make account" link under settings for the people that want it.

It definitely is some amount of dev work, but it's the kind of thing where you could make an acceptable version in a day or two. Especially if it just spits a secret code at you and you input that code into a different device. Password recovery isn't necessary. Privacy policy can be two sentences. Backups can be a daily rsync.