"I'm buying her a skirt with eighty pesos."
Translation:Le compro una falda con ochenta pesos.
The "con" here seems wrong. It sounds like the dress has 80 pesos. Why wouldn't the idea be that '"with" really means "by using" or some equivalent? Seems like we had that in a previous lesson. "Por" comes to mind...
for direct objects you use lo (M), or la (F) but for indirect objects you use le (M and F). In this case, 'her' is the indirect object. additionally, the plural versions are los, las for direct and les for indirect