There are three types of ‘barriers’ for novels:
Advertisements don’t provide Qidian International with enough money to keep going. Along with maintaining their site, they’ve got to pay everyone who works for them, including staff, translators and editors, the former two of which who usually make a living off of doing their respective jobs.
Some novels may have a Premium wall placed over it. This means that the novel will release a few regular chapters every week as well as even more ‘Premium’ chapters. These Premium chapters are like ad-walled chapters except you can’t watch ads to read them, you can only pay Spirit Stones. Read these links for more information:
https://forum.webnovel.com/d/2786-introducing-the-premium-program-beta-giving-you-advanced-chapters
https://forum.webnovel.com/d/3634-need-more-info-on-premium-program
https://forum.webnovel.com/d/2830-regarding-premium-what-it-means-for-readers
A lot of your questions should be buried in one these threads, mostly likely the middle thread.
Depending on the novel, the price varies, due to the number of words in it and other factors. Each normal, non-expirable Spirit Stone is worth USD 0.02. Bonus Spirit Stones are spent before Purchased Spirit Stones, however, the below list will only provide a price chapters, assuming that only Purchased Spirit Stones are spent: