"The bread costs ten coins."
Translation:Panis decem nummis constat.
«consto» traditionally demands the ablative case of the noun following it. Hence «nummis», the ablative plural of «nummus» is used here. Although when used with «quantus», the nouns that follow it can also be in the nominative plural, like in the course sentence: Quanti olivae et crustula constant? And I think «quanti» is in masculine plural here because it's ellipsing the plural of «nummus» (nummi), a masculine noun.
Panis is used here in the Nominative case -> use "panis" as shown in the Tips
Panem is in the Accusative case. Again from the Tips for Accusative case (e.g.):
(Singular) 3rd Declension//(Nom) mater//(Acc) matrem
So panis becomes panem