the only real difference time wise in reproducing the cake with one foam layer is
you don't have bake & fill the 8" cake
you still have to shop, measure, bake, make icing, decorate
it takes very little effort to also make/bake the 8"
in some cases acquiring the 8" dummy might mean an extra trip to another store even (gas, money, time)
so therefore a lot of people charge the same amount because it's basically the same amount of work
whatever your per serving price is
i think across the country i could see that going for $4-$8 a serving give or take by mortal bakers not the super big names
flowers might be additional--might be an upcharge for more than double amount of fondant too