Most cost-effective way of making an electronic "slot machine" with a hundred wheels?
How about using something mechanically similar to a combination lock?  In theory, you could get by with just one motor and no other solenoids or other such things.  Unfortunately, the time required for an update would be proportional to the square of the number of wheels operated by a single motor (since a full update of 100 wheels would require about 100 rotations clockwise, 99 counterclockwise, 98 clockwise, 97 clockwise, etc.)  On the other hand, if you were to split your device into ten sections, each of which could be connected or disconnected from the main drive motor using a solenoid, and allowed any or all to be driven at once, that would cut the update time by a factor of 100, leaving it pretty reasonable.

Addendum:

One could probably control a lot of wheels with a single motor, without the O(N&sup2;) spins requirement of the "combination lock" method, by giving each wheel a differently-notched cam.  There are many possible approaches one could take to doing so, depending upon what types of cams one could fabricate most readily.  If you have a convenient means of cutting flat sawtooth "gears" with various patterns of missing teeth, you could mount all the wheels to a common shaft with enough friction that the wheel would turn with the shaft unless blocked by a cam.  Each wheel would have a ratchet mechanism which would allow free travel in one direction, and would block travel in the other direction when a tooth was present (so any attempt to rotate the shaft would cause the wheel to slip on the shaft).  Use of two cams per wheel, along with a mechanism to globally engage and disengage a ratchet on one of them could greatly improve efficiency.