Though I do not know the specifics, I know that in China Qidian makes a special point of having the novels it hosts cost significantly less than those you would find in a traditional bookstore either online or brick-and-mortar. And yet on their international branch, Webnovel, they are charging what is approximately the same as a standard paperback.
Though the calculations were done by someone else, each Spirit Stone amounts to approximately 200 words. Each Spirit Stone cost about 2 American cents. According to Bing the average novel is between 70,000~120,000 words (let's go with the mean of 95,000.) That means that to get the equivalent content to a typical novel will require 475 SS, the lowest-priced option providing that quantity would cost about 10 USD. That would be around the price of a paperback from a traditional publisher, give-or-take.
But Webnovel does not provide the same level of quality control as a traditional publisher. I've personally seen many premium novels on the service filled with basic spelling and grammar errors (e.g. it's instead of its and vice versa) and to be frank it's embarrassing. Either the QC on premium novels should be tightened up, or the price of Spirit Stones should be reduced by, let's say, 30% to acknowledge the lower standards.
I don't mind the current business model. I'd much prefer to pay for what I read instead of having yet another subscription, but as it stands the value proposition Webnovel provides is seriously lacking.