OneDrive is pretty damn cheap _and_ comes with access to Office on the web. 1tb per family member, with Office? Goodly.

There's no official Linux client, but there are open source ones that perform well.

You could drop your mbox onto a OneDrive sync and then use fetchmail or similar to rip it out of your provider. Each email could be individually encrypted, so you won't have to deal with a 500gb blob to sync.