RE: Which foam trainer?
That $28 ARF is essentially just a foam wing, foam tail assembly, a stick fuselage, and some push rods. The NPS = no power system. This is a popular option, however, as all of the necessary components needed to complete the plane can be purchased at very low cost from online retailers like Hobby King or BP Hobbies.
You'll need an electric motor, electronic speed controller (ESC), two micro servos, a propeller, a NiMH or LiPo battery suitable for the prop, motor, and ESC, and a lightweight receiver (assuming you own a transmitter) in order to get your $28 ARF to fly. For less than $100, however, a GWS Slow Stick can be built to fly very, very well.