There are a variety of ways to set price
There is no one set way to do this. That you increased your price by 30% and saw no drop off suggests there is more to be taken, but to some extend by making a 30% price increase, you have limited how big a price increase you can make without affecting some perceptual bits
Part of why this is complicated is that we have a lot of cognitive biases involved in how we perceive value in this world. And how you signal value is also very subconscious - enough so that Nobel Prizes have been handed out over it
Testing, preferably adaptive testing.
Introductory subscription pricing doesn't usually reflect the quality of the newsletter. It's usually based on subscribers past subscription buying behavior.
What does matter is how many subscribers continue. The higher the drop-off rate, the less likely subscribers are interested in its content. Considering there wasn't any significant loss of subscribers due to a 30% increase, I would assume that this group is price inelastic during their renewal cycle.
My recommendation is to set up a two tier pricing system. Find or test a front end price (check with competitors) the price that will bring in the most amount of subscribers. List this price as an introductory offer and renew them at a 100% price increase rate. In other words, bring in as many tire kickers as possible and then punish your best customers.
Good luck and good selling.