Allison K pointed out a lot of the pros of VexIQ. It's really the only way you're going to be able to make something like
this for under $400.
However, no product is perfect, so I need to give fair warning that there are currently two HUGE issues with the VEX IQ kit.
1,) I often need to use my teeth to take pins out of places where they cant be pushed out with shafts. (doesn't seem to be easy to get a good grip on the pieces with pliers)
2,) Because such great force is required for putting in and taking out pins, your fingers start to hurt quite a bit.
These issues make construction a great hassle. Now, while this means I would never use this kit in an elementary school setting, the pros of a higher functionality ceiling might outweigh the cons of build convenience for a middle school environment.