Learn Spanish (Latin America): La computadora cuesta mil doscientos setenta y un pesos. - The computer costs one thousand two hundred seventy-one pesos.

In Spanish, you don't use un (a/one) before mil when saying "one thousand." It is simply mil. You only add numbers in front when you reach two thousand or more (like dos mil, tres mil).

How do you form numbers in the hundreds like doscientos?

To say "two hundred," you combine dos (two) and cientos (hundreds) into one word: doscientos. Notice that cientos is plural and must match the gender of the noun it describes. Since pesos is a masculine noun, we use doscientos. If we were counting something feminine, like computadoras, it would be doscientas.

Why does the sentence use setenta y un instead of setenta y uno?

The number uno (one) drops its final -o and becomes un whenever it comes directly before a masculine noun. Because pesos is masculine, setenta y uno changes to setenta y un pesos.

Why is setenta y un written as three separate words? Weren't numbers in the twenties written as one word?