"Navio+ is an improved version of Navio autopilot shield for Raspberry Pi A+/B+. It was designed both for your own custom robotic projects and as a platform for Linux version of APM (ArduPilot). Navio+ eliminates the need in multiple controllers onboard making development easier and increasing robustness."

But, at $168 the NAVIO+, $12 for the GPS antenna and $24 for the power module, it's three times the cost of a APM 2.6.