It's crazy to me that Infinity and Relay think they can get by on subscriptions, when the only people who use it enough to pay a subscription, use it more, making it exponentially more expensive as it's based on usage.
The Relay developer has acknowledged this and has been tracking the number of users API calls, looking at the 90th, 95th, 99th percentiles as he expects these to be the users who pay a subscription. To me it seems like he's done enough research to come up with an accurate price.
I've been a Relay Pro user for years and loved it, but I just don't use Reddit enough to justify another subscription. I was okay with a 1 off payment (especially as it was just a couple of quid, although I would've been okay with way more), but I just can't justify a subscription to Reddit for the value I get out of it.