We do use the plural. In the US, prices are usually said with "dollars" in the plural form. Ten dollars and fifty cents. In conversation, people will often omit "dollars" and just say ten fifty.
There is an important exception: number + unit of measurement + noun. Here, the number and unit are a compound adjective. In these cases, we use the singular. For example: a twenty-dollar bill, a 10-hour flight, a five mile radius.