the 450SA comes completly assembled except for the electrics. I have seen the 450SA combo in my LHS for as low as $289 and that includes the Align 430L brushless motor and 35amp esc!!! A very potent combo!

A T-Rex 600 can set you back upwards of $500 just for the kit without electrics. A decent battery with enough power to give you about 5 mins of flight would run you upwards of $150. A good motor and speed control can be very not cheap so an electric 600 without a radio and gyro can run around $900 just to get your feet wet.

As for stability, a bigger heli will naturally be more stable. Doesn't matter how well built a micro heli is. It will be inherently unstable due to the lack of mass and rotational stability. BUT a micro can be made to fly stable and behave with the right modifications to the flybar system.