Tanalei One flaw in your otherwise nice, though incomplete, analysis is your definition of "Fulls". Yes, technically 40,000 words is the starting point for a novel. However, in the markets that the books here are competing in, 40,000 is simply unsellably short. Authors in the field of fantasy and science fiction are told to target their books at 80,000 to 120,000 words - towards the longer end if there is substantial complexity and world-building going on. So your analysis is off by a factor of 2-3.
An example might be "Way of Choices" which I read on Gravity Tales and supported the author/translator by buying the Amazon novels as they came out. They published 4 volumes, each in the 100,000 to 140,000 word range, and priced at $3.99/volume (and then stopped - I would buy the rest if they ever published it!) That's about right.
My calculation (based on 10+ SS/chapter which is more the norm for the books I read here) is that the subscription is worthwhile as compared to SS purchase if you read 7 chapters/day (1.5 chapters free, and 5.5/day paid). It's worthwhile as compared to purchasing Amazon books if you read 18 chapters/day. It's never worthwhile when compared to subscribing to Amazon unlimited.
I don't think they need to match Amazon unlimited's price, but I think they should be within a factor of 2 if they want people to continue to subscribe. I'll undoubtedly subscribe for a couple of months once it looks like the software is stable. But it's not going to worthwhile paying that much for just daily chapter updates.