It's dollars per month. I have heard that Netflix is not charging enough for their ad-free tier to compensate the lost ad revenue.

Pricing an ad-free tier is a bit problematic. You would think that it should cost simply the ad-revenue per user more. However, since the more the ad free tier costs, the more advertisers would pay to reach those customers, the marginal ad revenue is higher. Someone on a hypothetical $30/mo ad-free tier has high disposable income, and is probably not being impressed by many ads already.