We have yet another phrase that involves a word that stems from chingar. And there's a lot more to come. But we don't want to get ahead of ourselves, let's decipher this phrase first.
To cut to the chase,"vales para pura chingada" means "you're not worth shit".
Vales comes for valor - to be worth something. And not only can you use it on people, you can use it in other contexts. Here are some more examples:
Este carro vale para pura chingada.
This car isn't worth shit.
Este pedazo de chatarra vale para pura chingada.
This piece of junk isn't worth shit.
As far as swearing in Spanish goes, this is pretty straight-forward. Or as the saying goes "pan comida".
¡Hasta la próxima!