Youtube's direct expenses are not published by Google, but there are a couple of ways we could measure it. One is the fact that Google is among the richest companies in the world, if not the richest at any given time. This definitionally indicates that the margins on their main revenue-generating services, among which Youtube is one, are extremely high, with revenue far, far, above expenses.

Another way we could measure it is by the value of an ad-view relative to the price of the subscription they offer. Ad views are auctioned and go for different prices based on category, demographics of viewers, etc., and aggregate statistics are not provided, but an ad-view typically tends to be in the range of US$0.01 per ad view. A subscription fee of US$9* to avoid ads, then, would require viewing 900 ads to justify the cost. I suspect in reality most people don't see more than 100 ads in a month, so Youtube is likely generating an 8x profit margin over costs of not showing ads to Premium users, give or take depending on how you work out the napkin math. If people had an option to buy an ad-free subscription with none of the other premium features for $1/mo, I suspect the uptake would be significantly higher and feel fair to the general population.

*After looking it up, Youtube Premium apparently actually costs US$14.

Anecdotally, I used to spend, I believe, ¥480 per month for a Niconico subscription (Niconico is the Japanese domestic equivalent to Youtube). I was content paying this subscription fee for years, until they increased the price up by 50% to ¥720, and about two years ago the price further increased to ¥990. I cancelled my subscription and stopped using the website. I am not opposed to paying subscription fees to platforms, but when it feels extortionate, I won't. The same is likely true for many or most people.