Description:

This is a small, light toy car accelerated by a slinky.  A rubber band is stretched across two fixed supports.  The car is placed against the rubber band, drawn back and released.

Physical Principle:

This toy demonstrates the principle of stored energy.   As the rubber band is drawn back, energy is stored and then released to propel the car after its release. 

Construction Cost, Durability, Safety

This is very simple toy.  Therefore, the cost of construction will be very low.  This, however, can vary depending on how complicated a car will be made (it can be extremely simple for this toy to work, but depending on visual-appeal needs it can be altered).  There are very few parts, the toy will be quite durable.  The most probably durability problem would be the snapping of the elastic strip.  The toy is also quite safe.