Thanks! We've been thinking of it, but rejected the idea. RPi A+ is quite light, so RPi+Navio weighs as the normal autopilot. Also, A+ is much cheaper.
Navio does NOT weight 24g. It's weight is around ~12g.
RPi A+ with Navio = 23+10 = ~35g.
Pixhawk weights 38g.
With Navio+RPi you get a lot of stuff compared to hardware like APMMini: network connectivity (LTE, long-range WiFi), affordable 1080p camera module with H.264 encoder for FPV, a lot of processing power for advanced flight algorithms (as an example - a lot of new features of APM such as EKF are NOT available on old APM platforms) and the overall hackability, which you don't get with any microcontroller based autopilot.
Not to detract from the novelty value of an RPi based flight controller, but if you were involved in the design, you must know that's just not true:
RPi A+: 23g, Navio: 24g, Total: 47g
APM Mini: 7g
That's a significant difference in a situation where every gram counts towards flight time.
The Navio also costs 6X an APM mini, and that's before you add the RPi and power module, which brings it to 8X.
I get why someone might develop this as a hobby project, but I can't think why anyone would buy one.