Like other posters have said, you can spend almost as much as you like but the price will depend on what you want.
Usually
the price will reflect not just the ingredients used but the time that
it will take to make. Also if you want something specific that nobody
else has asked for, then some suppliers may put an additional charge on
to cover any equipment that they may need to get. Fruit cakes can be
made in advance as the cake has a longer shelf life whereas sponge
cakes tend to be a bit more last minute, so a fruit cake may be
decorated 3 days before at a slightly less stressful degree, where as a
sponge may be baked one day, decorated the next (and depending upon the
complicatedness of it, may take well into the night) for delivery next
day.
As for sizings, again this will depend on what you
want and how you are going to serve it i.e. whether it will be served
as cake (and have a dessert as well) or use the cake as dessert. The
portions can be smaller if also having a dessert. Also, fruit cake
portions are usually smaller than sponge cake portions.
HTH
C 