"A cheap dress."
Translation:Un vestido barato.
I'm sure you found the answer, but here's my shot at this: 'Un' is masculine, and 'una' is feminine. When determining when to use 'un' or 'una', we usually look at the object of focus. For example, vestido, which means dress, ends with 'o', which is masculine. Therefore, un vestido is correct. Another example would be cartera, or purse, which ends with the feminine suffix of 'a'. The correct starting would be una cartera. Hope this helps anyone in this feed;