Personally, I find iPad Air a perfect tablet for its specifications, processing power and screen but, in terms of pricing I don't find it suitable for my next purchase. I'm looking for an alternative of iPad Air under $250, please do not recommend Nexus7.
You're looking for something that doesn't exist: an alternative to a 10" tablet that's under 250$. The only 10" tablet I can think of for less than that would be refurbished Surface RT.
I'll throw another one in for Nook HD+. I am writing this post on one... It was cheap and I run CM10.2 (its officially supported). It is NOT as smooth as iPad Air... But you can buy three for the price of an Air... What do you expect??