My 3 children all have summer birthdays, so we consume a lot of birthday cake in a short period of time. Cakes purchased at a bakery or the grocery store can be beautiful, but very expensive. Because all my kids are young (8 or younger), they don't care if they eat a homemade cake or a bakery cake.
Save the money and buy the cake mix and make it yourself. I bought my son's cake mix for $0.89 and the frosting for $1.58. The total cost was $2.47, rather than up to $40.00. The bonus to making it yourself is that your kids can help you mix all the ingredients and even choose the decorations for the top. My son wanted a cake with blue frosting, so I bought white vanilla frosting and added a few drops of blue food coloring. We added some sprinkles to the top as well.
You can also add some small toys to the cake too, like (new, clean) hot wheel cars for boys or Polly Pockets for girls.
