A toy I got at Half Price books on clearance for $2. One slight problem: the ‘runs’ are flat, so I had to improvise with coins to make inclines. Before you watch this, ask yourself, are you sure you have nothing better to do?
You may want to check out some more expensive but possibly working marble runs.
UPDATE: Apparently this very same kit is selling on Amazon for $35!
And others have had greater success:
Engineer Zero 