Let the streamer choose the price of their subs.
Amazon/Twitch keeps a fixed percentage of total income.
Prime subs stays the same.
So doesn't matter if the custom subscription costs 4$ or 100$, is like a fixed price, making the prime an even better option.
If the viewer wants to subscribe for 3,6 months or 1 year, he'll pay the current sub price multiplied by the ammount of months, allowing streamers to make sub promotions by their own.