If you lose your membership card at Bally Fitness, they charge you $10
to have another one made on the spot. How much does it actually cost
to make?

I imagine the total cost is not simply the 2" x 3" plastic card. 
There is the equipment involved... perhaps a camera, laminator,
printer... also:

The fact that people get a second card means that their first card is
out there somewhere.  Bally must have a good scanner with software
that allows them to remove the old card from their system.  And of
course Bally must provide the labor to create the cards and service
all the equipment.

I'm sure I'm missing some expenses involved, but perhaps you can see
where the $10 goes now.  It's not a stretch to argue that the
membership dues should cover the cost of lost cards, but Bally has
every right to choose the way it does business.