There was a time when we were considering such options. Not anymore. We provide the Hub functionality on four platforms: appliance, tvOS, macOS, and iOS.
We have already announced HomeKit support in 5.0 due later this year, and HomeKit (currently) only exists on iOS and tvOS (and watchOS). That means there will be an advantage for some users in the future to run Hub functionality on iOS or tvOS. The appliance, a Raspberry Pi port, a Linux port, a Synology port, or any of the many other possibilities we do get asked about, obviously cannot control HomeKit devices.
We hope Apple adds HomeKit this year to macOS. Regardless, non-Apple platforms will clearly never get HomeKit support. That may not matter at all to you. But if it does or could matter to you, your options are already iOS and tvOS. I consider iOS the strongest Hub platform personally. With the Guided Access feature locking out extraneous alerts, it works perfectly and there tends to be a very high correlation in our user base with spare iOS devices as users upgrade every year. tvOS is just as good functionally, but its lack of auto-launch and lack of a battery make it somewhat less resilient than iOS with Guided Access.
Granted, going iOS may not solve the “cheap as dirt” goal like a RPi, but that is a cyclical thing for most of our users. An effectively ancient iPhone 5S still runs iOS 10 and has been deprecated long ago from most user’s primary phone, and yet is a perfect Hub device. Hard to compete with free.