I can't provide information about the cheapest website to buy books online. Generally speaking, online bookstores would regularly launch promotions to lower the price of books. You can try to compare prices in different online bookstores or go to the local bookstore or library to borrow books. This can save money and enjoy reading.
Well, some popular manga websites include Viz Media and MangaFox. Viz Media offers officially licensed content, while MangaFox has a large collection but be aware of the legality of some uploads. It's always best to support official sources when possible.
AbeBooks can also be a good option. It's an online marketplace that aggregates books from various independent sellers. You can often find used light novels at very low prices here. The sellers on AbeBooks set their own prices, so with a bit of searching, you can come across some great deals. However, be aware of the condition of the book and the seller's reputation.
Well, websites like HentaiHere and Hentai2Read are often mentioned, but be aware that Hentai manga often contains adult and potentially inappropriate content.