Hola Marcos

When we simplify the answer with just saying the number, we normally make it "match" with the noun the number is referring to. In your examples, the common answer would be:

- Doscientas una

- Quinientos veintinuno.

The first one is referring to "personas", feminine plural noun, hence "doscientas una. If you answered fully you'd say: "hay doscientas una personas".

The second one is referring to a specific currency, because it's asking "How much do these cost? - so the number is going to be referring to either euros, pounds, dollars, etc, any currency. 

I assumed that would be euros, so you would need the number in masculine plural:

Cuesta quinientos veintiún euros. 

If you simplify it and drop the currency as you did in your example, then it would be "quinientos veintiuno" (because this way it acts as a pronoun so you need the full form of 21 = veintiuno)

We have some lessons on numbers that will give you some extra examples: 

Numbers with nouns

Numbers as pronouns

I hope this clarifies it. 

Un saludito

Inma