Zulu Alpha, the cheapest iPad Mini 3G (with GPS) is £369. If you already have an external GPS then you can get the wifi model for £269. It's still not as cheap as a Nexus 7 (a very nice device, but no Sky Demon), but it's much cheaper than a Garmin. I'm hooked on Sky Demon, so it has to be iOS for me (for flight planning - its only a backup for navigating)
I'm seriously thinking about buying an iPad Mini wifi (I have an external GPS) and selling my iPad 3 on EBay.