Congrats on the 1k subscribers! I personally would feel violated if i subbed to a newsletter and suddenly it was behind a paywall of $8 a month.
Brand New [0] did the same a while back, but for a much much lower price. And they offer their whole website with a massive history and daily updates for $2 a month or $20 a year.
I would personally try to find a sponsor instead of hiding your content that your 1k subscribers signed up for behind a paywall. A sponsor for a newsletter is IMHO a very normal thing.