Learn Spanish (Latin America): El viaje cuesta quinientos o seiscientos pesos. - The trip costs five hundred or six hundred pesos.

While many hundreds are formed by just putting the number and -cientos together (like seis + cientos = seiscientos), 500 is an exception. You just have to memorize quinientos. Other irregular hundreds to look out for are 700 (setecientos) and 900 (novecientos).

Would quinientos and seiscientos change if the currency was feminine?

Yes! The hundreds from 200 to 900 change to match the gender of the noun they count. Since pesos is masculine, they end in -os. If you were talking about a feminine currency like libras (pounds), it would be quinientas o seiscientas libras.

Why does the verb costar change to cuesta here?

Costar (to cost) is a stem-changing verb. This means a vowel inside the word changes when you conjugate it. For a he, she, or it (like el viaje), the o changes to ue, making it cuesta.

Why is it el viaje? I thought words ending in e could be masculine or feminine.