Via some Googling:
Here are the monthly and annual prices for CuriosityStream: $2.99 a month for standard HD streaming. $9.99 a month for premium 4K streaming. $19.99 a year for standard HD streaming.
Do you have to pay for Nebula?
For access to its library, which grows by around 250 new videos each month, it charges $3/month, or $30/year. Fees are used to cover Nebula's basic operating costs, and then are split, 50-50, between Standard and creators. Creators are paid proportionally, based on how much watch time their videos glean each month.
Does Nebula come with Curiosity Stream?
Most importantly, you still get complimentary access to Nebula with your Curiosity Stream subscription!
And this might be helpful to know:
What is the difference between Curiosity Stream and Nebula?
While Curiosity Stream has original content, as far as I can tell all of the Nebula stuff is also available on YouTube. So as a Nebula customer the only real benefit is watching content without ads. This is well and good, but ads aside, Youtube is simply a more convenient platform.
If you go to CuriosityStream's website
and scroll down from their homepage, they show the pricing.
Standard HD monthly: $2.99
Standard HD annual: $19.99 (best value/most popular, they say)
Premium (4K) monthly: $9.99
Premium (4K) annual: $69.99
Assuming everything I'm reading is accurate/up-to-date, you can get HD CuriosityStream for $20 a year and that includes access to Nebula's library.