Ask your mom if she feels you should be working for free? Essentially she's saying your time isn't worth anything. But she's also basing her idea of pricing on grocery store pricing.
If you give us an idea of the sizes of the tiers, or the number of people that they want the cake to serve, then you can get a better idea of cost. For example ,if you're looking for 74 servings, that would be 6/8/10 round and for me, would be $222 at $3/serving for buttercream. If the fondant accents aren't too intricate and are few, I might not charge for them, but otherwise, could add an addition $20-30 depending on what kind of bows and how long they take. And as Leah has said, delivery costs for sure. I don't know how Leah does her calculations, but if it's per mile, I would be charging the total amount driving to get there and back. If you're making a 6" smash cake, that is equivalent to an additional 12 servings (even though it isn't being served) and using the same pricing model, would be an additional $36. So, the cake + smash cake, before fondant decor and delivery is $258,
I think your mother needs an education on what a custom cake actually costs. If you went with her pricing, as someone else has said, I don't think you'd even be covering your costs. Besides your cake ingredients, you've got fondant, gumpaste, cake circles, drum board, dowels, box(es) plus your labour and utilities.