Most bakers will charge the same for a fake styrofoam tier in a wedding cake as real cake.  The reason being is that decorating a fake tier or real cake takes the same amount of time.  Your cost for a wedding cake is based primarily on labor to decorate the cake.  Ingredient cost is minimal.  Fake styrofoam tiers actually cost about the same as the ingredients for a cake.  
  One way you can save money is by purchasing a smaller tiered cake and then additional kitchen cakes to supplement your tiered cake.  For instance if you are having 200 guests, design a 3 tier cake for 100 and then have undecorated cakes served from the kitchen.  Tiered cakes are generally priced per serving, but the undecorated kitchen cakes are usually a flat charge and less money.   One suggestion to keep in mind is making sure  your  tiered cake does not look out of proportion to your wedding guest number.  For instance a two tier cake for 50 people  when there are 200 guests attending looks obviously small,  unless you are displaying a dessert/candy buffet in addition to the small two tier cake.  
   Another way to save is keeping your cake design minimal, choose buttercream which costs less than fondant and avoid intricate designs and details on your cake.  .