[aaronbrethorst]

How important is your tool to your users?

    teachers often give lectures from SageMathCloud
    or run computers labs

OK, sounds important.

    I care that users don't lose their data in case
    of a disaster (hackers or lightning striking
    Google four times), which just makes things cost
    more.

And it sounds like you care. So, how much do you charge for such an important tool?

    we have $490/month in recurring revenue (we have
    56 subscribers to various plans)

$8.75/month. Try tacking on an extra zero to all of your plans. Or, better yet, tack on an extra zero and ALSO let your customers decide whether or not they care about things like backups.

I don't want to sound like an asshole, but your business is never going to succeed if you keep going down this path. And to be clear: I want to see you succeed.

Here are a few things by patio11 you should go read right now:

http://www.kalzumeus.com/2014/04/03/fantasy-tarsnap/

https://training.kalzumeus.com/newsletters/archive/saas_pric...

http://www.kalzumeus.com/2012/08/13/doubling-saas-revenue/