@GrapheneOS posted their guestimate as "To do this for a backup app, it will take us dedicating around 2 experienced developers to it for a couple months to get it written and then continuing to put work into it for around a year for it to reach the point that it's a great app." which sounds to me like 'two developers for a year' give or take, and that's quite a bit more than $40K.
What's a better way to fund something like this; through donations to the general GOS project, or specifically for the app (how/who'd handle that?). Also, smaller $ amounts/month are easier to handle for most, in the long run.
Either way, a proper backup app would definitely be something I'd pitch in to make it a reality.